shimano to campy wheel question

I'm in the process of building up my first bike. I would really like
to build the bike with campy stuff (chorus and centaur mix). one of my
issues is that i've already got a shimano bike with an extra set of
shimano compatable wheels. I've got a set of ksyriums and a set of
cosmos. what i want to know is if it is possible to just replace the
part of the rear hub that the cassette goes onto (i'm not sure what
this is called, maybe the freehub body?) and that way the hub will
work with a campy cassette. Also i wanted to know if this is possible
would the part be interchangable between the ksyriums and the cosmos,
so that i could decide which wheel i wanted to use with the campy
bike? Thanks in advance and hopefully that wasn't to confusing.