I'm pretty sure Kirt (and his fellow transformations) from shadowcaster don't have sprites in the game to extract and they'd have to be made from scratch (except maybe Ssair), but it's a cool idea and it would be neat to see this kind of character in deathmatches. The mage can already possess other creatures, so Kirt's power wouldn't necessarily be unfair or game-breaking (but with multiple mana types it could be confusing how his mana drains/recovers in his forms).
I guess I really liked the music more than the actual game though. Too bad they're XMI sound and not MIDI tracks. It would be cool to hear the golems' fire world music in Hexen.