if there is a spacer behind the 7s cassette than you already have an 8 speed freehub.. if not you have a 7 speed freehub and it will not fit.

Disclaimer: someone might pipe up and say you can do it by building some sort of custom 8 speed cassette out of 9 speed sprockets and spacers or something but I don't recommend going through the trouble of something like that. Better to do it properly and change the freehub or buy a new wheel.