To edit your key, go:
Library>Application Support>MBWarband, and then in MBWarband there should be a document titled "serial_key", which corresponds to the Warband Serial Key, and, if you got NW, there should also be "serial_key_nw".
Just open either in TextEdit and you should see the key the game uses. Change it by pasting the one you have in Steam. Just go to Library in the Steam App, right click "Mount & Blade: Warband", and select the option "View CD Key".
Hopefully this will work!