I had tried doing something like that before (hoping it would be a simple feat), and the same thing happened. I figure that to do this, there's probably some sort of routine of variable that stops a pokemon from leveling up, and replaces it with a level 0 ???, if it's level is set above that through hacking, that would need to be removed/modified. I would try to search the game for such a thing, but I have no idea how people do stuff like that. I hope that some of the experts and more experienced hackers will take a look at this.