Fueled Spellfire with Divine Smite and Critical Hit.

SPELLFIRE ADEPT:

Fueled Spellfire. Once per turn, when a spell you cast deals Radiant damage, you can expend up to two Hit Point Dice, roll them, and add the total rolled to one damage roll of the spell.

Based on Divine Smite adding damage to the attack and Fueled Spellfire adding the total of its effect to the damage roll, I'd have to agree that the dice of Fueled Spellfire aren't damage dice for the attack, but rather function as a damage bonus (the same way a stat modifier or magic enhancement would).

I suspect you are right.

But then, when you cast Divine Smite, don't you also roll extra 2d8 radiant and add the total to one damage roll of your weapon?

It is a weird wording, but I hoped that the "add the total rolled to one damage roll of the spell" was meant just for avoiding the extra damage to be added to spells that do multiple damage rolls per round, like Divine Favor. 
I wish they had been a bit clearer TBH. 

Divine Smite:The target takes an extra 2d8 Radiant damage from the attack.

Fueled Spellfire:You can expend up to two Hit Point Dice, roll them, and add the total rolled to one damage roll of the spell.

The difference is subtle, but one adds dice to the attack, one adds the total of the roll to the damage. This distinction also shows in similar effects, like Sneak Attack or Hunter's Mark:

Sneak Attack:Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack roll.

Hunter's Mark: You deal an extra 1d6 Force damage to the target whenever you hit it with an attack roll.

Both of these effects add extra dice, where Fueled Spellfire calculates the total before adding to the damage roll. That's how I'd read it anyway, it's similar enough that I could see it going the other way (and I don't think the extra damage is going to be a huge deal, to be honest, it's only up to 2d12 at most if you're multiclassed in Barbarian and choosing to cast a spell). Plus it consumes a resource.

Not sure if it helps, but I had a doubt related to Fire's Burn, Frost's Chill and Critical Hits, in case some similarity can be found.

Also, from the Dev:

@quadhund If I make 1 attack with a longsword as a part of casting Green Flame Blade, while having the Hex spell affecting the target, and then I use Divine Smite once the attack hits, how many damage rolls are there?
@JeremyECrawford When something in the game (Sneak Attack, Divine Smite, hex, etc.) causes your attack/spell/etc. to deal extra dice of damage, those dice are added to the damage the effect is already dealing, if any. It's one big damage roll, extra damage included. 
@DaveWil33 Would only be a second damage roll if there was something locked behind a save like poison damage, yes?
@JeremyECrawford If an attack has a damage roll but also a second damage roll (not extra damage) that is contingent on a saving throw, the damage of that second source is a different damage roll from the first.
@RubiksMoose how do you distinguish between second damage roll and extra damage?
@JeremyECrawford Separate damage rolls are delivered by separate attack rolls, saving throws, or other processes. Extra/bonus/additional damage is called extra/bonus/additional damage.

Both of these effects add extra dice, where Fueled Spellfire calculates the total before adding to the damage roll. That's how I'd read it anyway, it's similar enough that I could see it going the other way (and I don't think the extra damage is going to be a huge deal, to be honest, it's only up to 2d12 at most if you're multiclassed in Barbarian and choosing to cast a spell). Plus it consumes a resource.

It makes sense. I am not completely convinced the wording was done on purpose though :D Would there be any way to ask the devs directly?

@quadhund If I make 1 attack with a longsword as a part of casting Green Flame Blade, while having the Hex spell affecting the target, and then I use Divine Smite once the attack hits, how many damage rolls are there?

@JeremyECrawford When something in the game (Sneak Attack, Divine Smite, hex, etc.) causes your attack/spell/etc. to deal extra dice of damage, those dice are added to the damage the effect is already dealing, if any. It's one big damage roll, extra damage included. 
@DaveWil33 Would only be a second damage roll if there was something locked behind a save like poison damage, yes?
@JeremyECrawford If an attack has a damage roll but also a second damage roll (not extra damage) that is contingent on a saving throw, the damage of that second source is a different damage roll from the first.
@RubiksMoose how do you distinguish between second damage roll and extra damage?
@JeremyECrawford Separate damage rolls are delivered by separate attack rolls, saving throws, or other processes. Extra/bonus/additional damage is called extra/bonus/additional damage.

If I interpret it correctly, that would mean that the spellfire damage is doubled with a crit. 

Oh no, The confusion intensifies :D

Well, if you consider it's a second damage roll that is contingent on resolving a Saving Throw or a spell attack to determine whether you're dealing Radiant damage, the damage from this second source (Fueled Spellfire) would be a different damage roll from the first one.

Quote from Jayce_Danathar >>

Both of these effects add extra dice, where Fueled Spellfire calculates the total before adding to the damage roll. That's how I'd read it anyway, it's similar enough that I could see it going the other way (and I don't think the extra damage is going to be a huge deal, to be honest, it's only up to 2d12 at most if you're multiclassed in Barbarian and choosing to cast a spell). Plus it consumes a resource.

It makes sense. I am not completely convinced the wording was done on purpose though :D Would there be any way to ask the devs directly?

@quadhund If I make 1 attack with a longsword as a part of casting Green Flame Blade, while having the Hex spell affecting the target, and then I use Divine Smite once the attack hits, how many damage rolls are there?

@JeremyECrawford When something in the game (Sneak Attack, Divine Smite, hex, etc.) causes your attack/spell/etc. to deal extra dice of damage, those dice are added to the damage the effect is already dealing, if any. It's one big damage roll, extra damage included. 
@DaveWil33 Would only be a second damage roll if there was something locked behind a save like poison damage, yes?
@JeremyECrawford If an attack has a damage roll but also a second damage roll (not extra damage) that is contingent on a saving throw, the damage of that second source is a different damage roll from the first.
@RubiksMoose how do you distinguish between second damage roll and extra damage?
@JeremyECrawford Separate damage rolls are delivered by separate attack rolls, saving throws, or other processes. Extra/bonus/additional damage is called extra/bonus/additional damage.

If I interpret it correctly, that would mean that the spellfire damage is doubled with a crit. 

Oh no, The confusion intensifies :D

No, because you aren't adding the dice to the damage of attack. You are adding the result of the dice to the damage of the attack.

From Sage Advice: https://www.dndbeyond.com/sources/dnd/sae/sage-advice-compendium#CompiledAnswers
For the College of Valor, does using Combat Inspiration’s Offense feature mean that the Bardic Inspiration die counts as a damage die for purposes of Critical Hits?
Yes.

The wording of Spellfire Adept is closer to Valor Bard Combat Inspiration than the wording of Sneak attack: both say to add the roll to the damage, and it was ruled that the Combat Inspiration dice would be doubled on a crit. 

VALOR BARD COMBAT INSPIRATION:
Offense. Immediately after the creature hits a target with an attack roll, the creature can roll the Bardic Inspiration die and add the number rolled to the attack’s damage against the target.

I think the confusing part is that they worded it so as to only affect one attack of a spell that has multiple attacks, but being that these are worded so similarly in that the total is added to the damage, I think it should be interpreted that Fueled Spellfire should work this way as well.

Mmm. But it's not exactly the same, right? 

- Fueled Spellfire: when you deals Radiant damage ...

- Combat Inspiration’s Offense: when you hit a target ...

I mean, it's like Athanar90 said in the first reply:

Quote from Athanar90 >>

Based on Divine Smite adding damage to the attack and Fueled Spellfire adding the total of its effect to the damage roll, I'd have to agree that the dice of Fueled Spellfire aren't damage dice for the attack, but rather function as a damage bonus (the same way a stat modifier or magic enhancement would). 

Quote from Jayce_Danathar >>

Both of these effects add extra dice, where Fueled Spellfire calculates the total before adding to the damage roll. That's how I'd read it anyway, it's similar enough that I could see it going the other way (and I don't think the extra damage is going to be a huge deal, to be honest, it's only up to 2d12 at most if you're multiclassed in Barbarian and choosing to cast a spell). Plus it consumes a resource.

It makes sense. I am not completely convinced the wording was done on purpose though :D Would there be any way to ask the devs directly?

@quadhund If I make 1 attack with a longsword as a part of casting Green Flame Blade, while having the Hex spell affecting the target, and then I use Divine Smite once the attack hits, how many damage rolls are there?

@JeremyECrawford When something in the game (Sneak Attack, Divine Smite, hex, etc.) causes your attack/spell/etc. to deal extra dice of damage, those dice are added to the damage the effect is already dealing, if any. It's one big damage roll, extra damage included. 
@DaveWil33 Would only be a second damage roll if there was something locked behind a save like poison damage, yes?
@JeremyECrawford If an attack has a damage roll but also a second damage roll (not extra damage) that is contingent on a saving throw, the damage of that second source is a different damage roll from the first.
@RubiksMoose how do you distinguish between second damage roll and extra damage?
@JeremyECrawford Separate damage rolls are delivered by separate attack rolls, saving throws, or other processes. Extra/bonus/additional damage is called extra/bonus/additional damage.

If I interpret it correctly, that would mean that the spellfire damage is doubled with a crit. 

Oh no, The confusion intensifies :D

 No, this is being added to "one damage roll of the spell".  You would resolve the critical hit first then add this damage.  If was added to the total damage of the spell it would double.  What they should have done is simply made it a hard number, those are never doubled by crits.

Quote from Tarod_ >>

Mmm. But it's not exactly the same, right? 

- Fueled Spellfire: when you deals Radiant damage ...

- Combat Inspiration’s Offense: when you hit a target ...

 Is there a difference? :/ If you hit you also deal damage right? 

I was convinced that the Fueled Spellfire damage wouldn't be rerolled, but I was not aware of the Valor Bard Combat Inspiration’s Offense. Maybe I am missing something but they look identical to me on technical level. 

Quote from Jayce_Danathar>>

Quote from Tarod_>>

Mmm. But it's not exactly the same, right? 

- Fueled Spellfire: when you deals Radiant damage ...

- Combat Inspiration’s Offense: when you hit a target ...

 Is there a difference? :/ If you hit you also deal damage right? 

 Usually yes, but technically... 

[...] 3. Resolve the Attack. Make the attack roll, as detailed earlier in this chapter. On a hit, you roll damage unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage. [...]

So for example, after hitting, but before rolling damage, you could cast a Smite spell to add extra damage to the attack (doubled when landing a Critical Hit), EDIT: or use what we were commenting on, Combat Inspiration's Offense.

Or, once you know you've been hit, you could cast Shield to avoid rolling damage.

For this reason, I think Fueled Spellfire can be applied after the attack hits + damage is rolled (whether it's a critical hit or not), but not, say, in the "middle" of that process.

This topic is becoming quite relevant, since the upcoming Evocation Adept feat appears to work in essentially the same way as Spellfire Adept. And if you use them together on a crit you could make considerable damage (8d6, 28 more damages on avg, or  8d10, 44 more damages on avg if you are a paladin). 

There is also a new UA Wizard subclass, the Imaskarcanist, which at level 3 can convert elemental spell damage into Radiant damage, making this interaction potentially much more common.

One thing I wanted to point out:

Fueled Spellfire. Once per turn, when a spell you cast deals Radiant damage, you can expend up to two Hit Point Dice, roll them, and add the total rolled to one damage roll of the spell.

The condition is simply that a spell you cast deals Radiant damage. It does not require an attack roll, so the feature can also work with spells that deal damage through a saving throw.

The feature also does not seem to specify exactly when the Hit Point Dice must be rolled relative to the spell's normal damage dice. As far as I can tell, you could roll the Spellfire dice first and the spell's damage dice afterwards, or vice versa, without changing how the feature functions.

The Critical Hit rule says:

Critical Hit

If you roll a 20 on the d20 for an attack roll, you score a Critical Hit, and the attack hits regardless of any modifiers or the target’s AC. A Critical Hit lets you roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers.

If I recall the original argument correctly, the reason some people consider Fueled Spellfire/Fueled Evocation not to double on a Critical Hit is that the Hit Point Dice are supposedly not themselves "damage dice." Instead, you roll them first and then add the resulting total to the spell's damage roll, so the result is treated as a modifier.

I can understand that interpretation.

However, I think Combat Inspiration: Offense makes the issue considerably less clear:

Offense. Immediately after the creature hits a target with an attack roll, the creature can roll the Bardic Inspiration die and add the number rolled to the attack’s damage against the target.

This seems important because Offense also does not say that the Bardic Inspiration die deals "extra damage" or explicitly call it an additional damage die. It tells you to:

"roll the Bardic Inspiration die and add the number rolled to the attack’s damage"

That construction is extremely similar to Fueled Spellfire:

"roll them, and add the total rolled to one damage roll of the spell"

Yet Sage Advice specifically rules that the Bardic Inspiration die from Combat Inspiration does count as a damage die for the purposes of a Critical Hit.

So I don't think the fact that Fueled Spellfire tells you to roll the dice and then "add the total rolled" is, by itself, enough to establish that those dice are merely a flat modifier rather than damage dice.

If Combat Inspiration's Bardic Inspiration die can qualify as one of the attack's damage dice despite being worded as "roll the die and add the number rolled," then it seems reasonable to ask why Fueled Spellfire's Hit Point Dice should be treated differently simply because it says "roll them and add the total rolled."

That doesn't necessarily prove that Fueled Spellfire/Fueled Evocation doubles on a Critical Hit, since the two features are not worded identically. But I think the Combat Inspiration ruling makes the argument for doubling them considerably stronger, and at the very least suggests that "add the total rolled" does not automatically mean "this is a modifier rather than damage dice."