The Kickr Core comes with an 11 speed freehub and an additional spacer to allow the use of 8-10 speed cassettes. Traditionally a 7 speed cassette can be used on an 8-10 speed freehub with the addition of a 4 or 4.5mm spacer.
https://problemsolversbike.com/produ...pacers_-_24987 I don't know any reason you couldn't use your 7 speed cassette plus these 2 spacers on the Kickr but might be worth asking Wahoo. Edit- oops, I see you said "freewheel." If you do have a freewheel (vs cassette,) you'll need to buy a cassette as well.