You don't need to register TW2 - the DRM of the retail box was stripped with the first patch and the GoG version didn't have any DRM in the first place. You should be able to receive all DLC's for the game no problem. You should just make sure that your game is patched to the latest version - manual patches are here, but your launcher will probably do that by itself, if you let it. Have fun with TW2!