You're doing nothing wrong.
The GP3 track editor always only shows the software jam and folder names.
Just reassign all of the hardware jam files that you need to with the track editor, but use software jam names and and directories, ignore the error messages that you'll get, and save the track dat file when you're finished. Be sure to backup the track dat file, just in case, and then insert magic data into the track dat file before you try to load the track in the game.
It will work.
Trust me. I've done it many times.