Yes it is. How do you think gjoerulv did it?
You'll need to use both Hojo and Proud Clod (unless you feel like editing scene.bin the old fashioned way (like I did, back in the day

). The only hitch is that you will need to use an existing model for the enemy, or replace one that is already there.
Come to think of it, if you want to create a new enemy with a new
appearance, then you might have trouble doing it from scratch; I'm not sure whether the game will accept new enemy models, but I can't see why it wouldn't...