Oops..tar is supported for cpmove-user files, didn't it work?
To be honest I didn't try importing any .tar files since the first post gave this ominous warning "If you have user.tar.gz instead of cpmove-user.tar.gz, it will not work! pkacct forms cpmove-users.tar.gz. If you have a list of user.tar.gz files, and want to rename them all in current directory"
I assumed since the filename was strict about the filename, it would probably be stricter about the file format. Of course I should never assume.
So, that's on me. My bad. I'll check .tar restores on the next batch I restore.