Is there a way you could make zerglings be able to evolve into Ultralisks similar to how Hydras evolve to lurkers?

Yes. In Firegraft, copy one of the Mutalisk's morph buttons, paste it onto the Zergling's button set, and change the action variable to Ultralisk. Keep in mind that if a player morphs the ling then cancels the morph, they'll get a Mutalisk...

but there's no way to make them get a zergling back if they cancel?

Not that I know of. Unless... How many morphing units do you have in total, and what are they?

currently i just have the default morphing units...

Anyway I had to make a new button set because there was no zergling one (it uses Basic Burrower) so i based it off that and added in Lurker Aspect, changed the icon and Action Variable to Ultralisk. It doesn't matter what the action or requirement strings are, right? Anyway, I made Lurker aspect not require it to be researched then i changed the zergling to make it use the new zergling button set, and then ran the game with my changes, and the button showed up, but when i click, it does nothing.

You need to edit the DAT Requirements. I know you'll need to edit Orders, and maybe the Lurker and/or Ultralisk requirements.

what order would I need to edit?