He isn't asking to be able to use his old CD keys to get free EE. He's asking if, in case he buys EE, he can use his old CD keys to play the game instead of new ones that come with EE.
He's asking because he still wants to access his old MP characters/accounts when switching over to EE as they're tied to old CD keys. You can type in any name you wish when connecting to a server, a little window pops up asking for one. I assume that if you use the old account name and password (I assume it will ask for one if you are using a registered account name on that server) associated with that former account on the server that things will work just fine. I don't see anywhere to input a cd key on the steam version but I do remember there being a text file in the game folder somewwhere that had those listed for the game to reference when starting up.