BK, like most of Rare's titles, uses a different microcode variation. SubDrag's dealt with BK much more than I have though, so try catching him at http://www.therwp.com/forums/
for specific data on embedding files.
Mumbo's Wand is mostly dedicated to stage design, but that said you could probably use the porting tools and a hex editor to take an imported model and replace a global character model with Mario. Just don't expect him to use his own moveset or anything. Animation formats are annoying to say the least.
(hoping those features are in the public beta now...)