The way we did it is to create additional user accounts for the mailboxes that weren't the user's primary, and to give that user access to the other mailboxes.
A better way to explain this is to say that the user had mailboxes A, B, C, and D. A was his personal, and B, C, and D were mailboxes related to specific topics (Postmaster, mailing lists, whatever.) We created different user accounts for B, C, and D and just gave the user full access to those mailboxes.
For additional security you can disable the user accounts afterwards, but you will get informational messages in your event viewer that a disabled user has received mail.
Hope that helps.