cj and me are right bolt, =P,
i made a duke nukem model that had a hand grenade, shotgun, and 2 pistols all with seperete bones stemming from the spine bone,
they would move with the spine bone in all the existing animations, (if you placed the models on the waist they would stay there but they would rotate with the waist,if the bone they stemmed from is on the waist,, which is good)
for custom animations you just hafta animate the new bones just like you would any other one,
some bones dont allow you to add bones though, or it could be entire skeletons,
but when i used the vegeta skeleton i was able to add 4 bones and animate them all seperately without having to redo all the animations except for the ones i wanted to change,
if you want ill send you the model as proof, and somewhat of an explanation to how i did it,
but i think i explained it good nuff there to tell you how,