Once you've consumed the CD key from a retail title by associating it with a Steam account, you can't use it again.
Furthermore, Steam will not allow you to merge accounts or move keys between accounts:
Steam accounts can not be merged together.
Per the Steam Subscriber Agreement, Steam game subscriptions / CD keys are nontransferable and cannot be reset / moved between Steam accounts.
The only exception that I've ever found is Company of Heroes, where I've been able to give away my account keys to other people, even those purchased on Steam. They limit you to 3 transfers, however.