The game engine of FF7 is quite stretched for it's day. In other words they pushed it quite hard.
Although possible to do (add new enemies) you need to do several things for this.
1) create a new enemy model.
2) create animations for this new enemy (there are several default animations you must have).
3) create a new ID for the enemy
4) Add AI to the new enemy for any specific attacks this requires script coding.
5) Add the enemies stats and attack information.
6) Add the special attacks into the MAGIC information.
This is just off the top of my head. I don't think it's practical at the moment. Maybe when Q-gears is much further along shall we say?
Cyb