Picked up the Kickr 2nd hand without thinking about the cassette etc. details. Not too fussed about having all 11 ratios on the trainer so wondering if installing a spacer or similar will work or do I need (and can I) to fit an 11 spd cassette to the wahoo?
Presumably the Kickr has a '10 speed' 34.95mm freehub body rather than a '11 speed' 36.75mm one.
Some wider-range 11 speed cassettes come with a 1.8mm spacer and will in fact fit on a 10 speed freehub body. The larger sprockets allows the spider to overhang the inboard end of the freehub body without the sprockets interfering with the spokes (which you of course don't have on the trainer).
Because the Kickr will adjust resistance on the fly for you (in ERG mode), I've found it possible to run the wrong derailleur with the factory 11 speed cassette. Just find a gear where it is lined up pretty well and don't shift. Shifting won't matter anyway, because if you shift to a lower gear to spin, the Kickr will just increase the resistance. You can also adjust resistance manually from the companion app or an Elemnt bike computer to control power/cadence.
As mentioned by Andy in the comments, you may have issues working with apps that automatically control resistance such as Zwift or RGT Cycling.
Not sure what you are getting at with a spacer for the conversion...the indexing would be way off because an 11 speed cassette has the cogs closer to each other than a 10 speed (that's why you can use an 11 speed chain on a 10, but not vice versa).
As far as installing an 11 speed cassette, that should be fine (it's actually stock). You can even get it straight from Wahoo if you like: https://www.wahoofitness.com/devices/accessories/11-speed-cassette.