You mean rehubbing a wheel? Yeah, done that loads.
If you mean fitting an 11 speed freehub to an existing hub = massively dependant on the hub fitted. You'd need to know model and year.......
Or if you mean sticking an 11 speed cassette on an existing hub/freehub.
Campag = easy, fits straight on in most cases.
Shimano MTB = easy fits straight on
Shimano road = no, not really, there is one cassette that'll fit (and it's wide range)