There are really two questions here
A) Can I have one edition (fluxbox) in another (XFCE)
No these are two separate "systems"
B) Can you install fluxbox (proper)
Yes any DE can be installed on any system. That said fluxbox is really different (not really a DE I think) so I don't know if there are any hurdles
Don't fix it if it ain't broken, don't break it if you can't fix it