It took a while but I finally found a way to register a new user and get him connected to the existing mailbox.
- Open Exchange System Manager and navigate to the old mailbox. Right-click and select Delete. It's okay, it won't really be deleted, it just gets flagged for deletion. It would have to sit like this for at least 4 days before it really gets deleted, unless you have changed the interval from the default. The important part is Exchange will remove the existing ownership information to you can reassign it to another user.
- Register the user from the Exchange server. Go through the setup as you would for any other user, including creating a new mailbox. You need to create it in a different message store than the existing one, though.
- After the user is registered and the processes have run to populate his SMTP and X400 addresses, right-click the user and delete his mailbox. This time it will really delete his mailbox, and that's okay since it was just temporary.
- Back in Exchange System Manager, right-click the old mailbox and select Reconnect. Enter the user's account name and click OK.
- Give it about 10 minutes you should be able to get to the user's existing mail.