I get that the 11s freehub in the conversion kit is going to be longer than the 10s one it's replacing, but is it a normal 11s freehub (and the end-cap or some other part supplied with the kit is special), or a special '11s freehub designed only to work on a 10s hub shell'?
You get a new, longer freehub to fit the extra cog. There's a video on YouTube of a guy doing the conversion