Well, that is not possible.
A mmo usually have rules that account sharing is not allowed, so either you will have to use the old user info, or buy the game yourself.
There is also another factor, if the person that gave you the product key already had an account bound to that product key. You will however not be able to use it.
If you do have a valid account, i would recommend that you head over to Account Management and check if there is anything you can change there.
I hope this helped you.
Snow.
