Philbo,

If the users are using Outlook for their client, Windows mail can be
configured to access multiple mailboxes on an Exchange connection.
This is done by going into the Mail configuration in the Control Panel,
and editing the properties to the Exchange connection.  On the first
page that comes up when you "Change" the Exchange connection, click on
"More Settings", then click the "Advanced" tab.  The top of that dialog
is where you'll enter the other mailboxes to open.

Another scenario (viable with any client), if the users don't need the
ability to choose which mailbox to use for their outgoing mail, you can
have Exchange forward out all the incoming mail from all the mailboxes
to one primary mailbox for the user to interface with.  I realize this
is a cheap "bandaid" fix, but it would get the job done.