You should be able to manually add the mailboxes in Active Directory
Users and Computers - right-click on a user and select Exchange Tasks.
Create Mailbox is the first option, and the one you would want.
How many users do you have? If you only have a handful, you should be
able to do this manually in short order. If you have a lot of users,
I'm sure you could write a script to do it, or find one that someone
else has already written.
- RAM