I read your post on the other board/thread, but I take it your asking on how to import it into Halo. Short answer: You can't until Sapien/Guerilla comes out. Long Answer: Yeah, but it must have the exact same number of vertices and indices as the real frag grenade, which I can see it doesn't.
EDIT: Btw, no offense dude, but all it is is two squares at different angles. I would HATE to play with something like that. Comon, its Halo, put some effort into it!