Definitely add another service account (at the very least)... However I would advice to go as far as create another Square Enix Account. Since you are to pay for another subscription anyhow. Having a separate Square Enix Account allow the second person to add in PS3 platform without worrying the SEN account tied to only one SQEX - forward thinking.

And yes, unfortunately, you will not be able to move existing character to new service account or SQEX.