Extend spell and modified spells

5.5e has a few class ability to modifies a spell.  Like the 18 level from Draconic Soc that lets you Summon Dragon from 1 min that does not need concentration.  I was just thinking this would be a great one to use Extend on to make it a 2 min (not a 1hour and 1 min) spell or any other ability like this.  Personally I would rule that it can be done as the meta is done after the spell is modified. 

What do you all think and what ability like this from the Soc or multiclassing would be the best for this?

I see no reason why not. The features like this don't specify that you can't use metamagic, so the same rules for using it should apply.

Whenever something says "cast a spell", you treat it like a normal spell you can cast.  So yes you can use a metamagic on it.

You can also use a metamagic on spells from scrolls, Enspelled Staff, Wand of magic missiles.

However, anything that says cast a spell also means you cannot do a Bonus Action Misty Step while using that class ability, a scroll, or the Wand of Magic Missiles.

Quote from Mog_Dracov >>

However, anything that says cast a spell also means you cannot do a Bonus Action Misty Step while using that class ability, a scroll, or the Wand of Magic Missiles.

Why wouldn't you be able to cast Misty Step?