SeizeTheDay

Let me explain. My friend wants to make a catfighting game, filled with special mechanics like belly punching, breast punching, bear hugs, all of that. He's unsure of what software to program this game in. I suggest MUGEN, but he's not sure, but I'm pretty certain I've made the right choice there - MUGEN should be infinitely moddable, it's in the name after all. So I ask: how would one go about modding MUGEN to give it those kinds of mechanics?