y'know, this is exactly why there's an entire section for nothing but tutorials. try checking there next time.
but, here's a link to my guide for model importing/swapping.
http://forums.kc-mm.com/index.php?topic=9248.0this will help you get the model working. if you want to port an entire character, and not just the model, that would require exporting all the animations from the FitCharacterMotionEtc.pac and importing them into the replaced character's MotionEtc file. then, you'll have to copy and paste everything from one PSA file to the next. it would take a long time, and there may be some unexpected glitches and stuff that may come up.
and from the sounds of it, you just took Luigi's model and put it on Pit. there's a large bone count difference between them, and that's why it froze on you.
hope this helps, and feel free to ask questions if you need to.