You can not add an extra slot if you already have 12 slots on a server.
Here's a quote from the patch notes saying subscribers are LIMITED TO 12 CHARACTERS PER SERVER
Actually you can.
I bought another character slot and now have 12/13 active Characters (meaning 12 of 13 possible characters) on my main server, as well as an additional 13th slot on the other servers. Subscribers are limited to 12 slots without buying additional slots.
ETA: You have to claim the item from the Cartel Shop and then use the item in your inventory. This opens the slot. I think I did a restart of the game to make sure it is shown correctly.