I don't follow you.
Besides you can convert j2b to .it format with OpenMPT. I just doubt that converting j2bs back into .it brings back any higher quality. The same way you can convert MP3s (once converted from WAV) back to WAV format, but it won't get back it's original quality, it was lost once it was converted into MP3 and that file doesn't contain any information which could help bringing it back... Or maybe I'm confusing and wrong?