It actually would be possible to port the CS+ challenges the other way around to the freeware version, it's just that nobody has decided to do this because the practical use would be very narrow, and the legality of distributing such ports is questionable. It wouldn't be too hard, though. You'd just need to appropriately match some rooms with certain configurations to ones used in the CS+ challenge so that the mapdata in the EXE will line up with said challenge's stage.tbl file, change the filenames of some of the maps to those used in the CS+ challenge, and replace the files in the freeware installation's stage folder with those of the CS+ port. And also do any necessary graphic editing.
In the case of Wind Fortress, though, fully replicating it would be problematic since the Curly clones and G-Clone boss are entities exclusive to CS+.