You can buy a second "key" to use (ie account) either by going to a store and buying a box with the disc's, or you can go to the PlayNC site and buy a key for it online. Here's the link:
http://www.plaync.com/store/gw_purchase.html
Just click on the bottom link to buy from PlayNC.
Once you have bought the new key.. I believe you can add another key to your pre-existing account.. log-into your account then click "Edit Account" .. I believe you can add cd-key's there for more accounts, but I'm not sure as I only have 1 account..
sorry I'm not too sure on the specific's, but I am sure that you are allowed to own multiple accounts at the same time. Just remember though, you won't be able to share storage between each "account" - each "account" is a seperate account with 4 character spots and it's own storage space.