Sorry, have to disagree. The answer to the question is to copy to composer, save your registration changes in the first 4 panel memories, and then copy back to custom.

The changes are then permanent, all RT1/RT2/L voice selections, balances, tempo etc are saved in the Custom .CST file with no need to save panel memories etc. In fact edited sounds, programmed user pads etc are also saved in the .CST file so there is no need to save them either.