I think i found a way to put your characters under a different gamertag, in the pso udata folder, this may be different depending on the slot you chose, copy PSO_CHARACTER_00 and rename it to PSO_CHARACTER_01 and paste it over the original 01 folder, it can be 01-03 depending on the empty slot you want, 00=slot 1 01=slot 2 etc, then in the game select the empty slot you pasted over and pick your new tag. If done right your characters shoud be under the new tag. I need someone to test this considering i've only done this using the same tag and not a different one.

i had this problem when my gamertag expired and signed up for a new one. yes this works.