Well. This has generated a lot more discussion than I would have thought.

I'm really not sure how hard this would be. Thing is, guys are actually enemies. You ever notice that enemy IDs start at 20? The guys are 1-10. So, most likely, you'd create new guys in the enemy editor. That much is fairly easy to do.
Here's where it gets difficult: the code was written with the assumptions that guys and other enemies could be distinguished by their IDs and that guys' IDs would never be greater than 255. Neither of those would be true anymore, and I don't know how hard it would be to deal with that. It can certainly be done, but it's not trivial.