So my DM has created some custom magic items for our Curse of Straud campaign that he is running via Fantasy Grounds, but I like to keep my PC up-to-date here. My PC now has a

"... ring of Arcane Endurance" ...."As gameplay, this ring increases the number of spell slots for each level (that you have) by one."

So my question is how do I make this here and have it add the extra spell slots or is that even possible given the current options?

It is not currently possible given the options available. 

Pearl of Power is the closest thing I think.

edit: Sorry, didn't read clearly. You get a spell slot for EACH level of spells you have. That's cool and OP =)

There is currently no such mechanic.

You might be able to make a feat similar to magic initiate. This would be a dirty and unorganized way of handling things but at least it would give you some tracking on your character sheet. :/

EDIT: From there you can make a magic item that grants a feat.

You can't achieve what you're after. Best you can do is create magic items with charges to represent the extra spell slots. It'll be clunky, awkward, and they'll have to keep swapping the magic items every two levels and you'll need to make a lot of them. That's the best you can get.

Quote from Scrotally >>

EDIT: From there you can make a magic item that grants a feat.

 
Feats cannot be granted by Magic Items, AFAIK. There's a feat modifier but it doesn't work that way.

Cybermind is right.

I thought maybe you could alter a ring of spell storing but it seems its effects arent actually implemented and it really just acts as a placeholder. Anywhoo I think modifying a magic initiate feat is a good way to go because you can at least easily alter the chosen spells from the manage feats tab. Then maybe just make a ring to act as a placeholder for your own organization?

Also this probably belongs in Homebrew rather than general

I was going to suggest you get a ring of spell storing when I read the headline.

Did not realize you were describing the most powerful mage Artifact ever created.  No caster has more than 1 eighth and 1 ninth spell slot.

The closest I know of is to take one each of all 9 of the spell gems, excluding the cantrip one.  But each requires attunement.  That would be 3 legendary, 3 very rare, 2 rare and 1 uncommon.

Thanks all for the feedback. It will be awesome, provided I live. As well, I kind of know what the ring does, but I have to let the DM know the first time I trigger it to use one of the spells....in Barovia, so it might kill me as well. Or Strahd might. Or maybe we'll all live. Anything is possible. Thanks again.