You can't install an 8-speed Campagnolo cassette on a 9/10/11 speed freehub. The spline patterns are different. But you can 'roll your own' 8 speed cassette using 8 of 9 cogs from a 9-speed cassette and a Shift-8 spacer kit from Wheels Mfg.

Branford Bike's web site has good info and pics re: Campy cassette and hub compatibility:

Cassettes and Cogs - Branford Bike - Seattle/Bellevue - Campagnolo Pro Shop