You could just have each user log into their own mailboxes and change their Personal Operator to the extension number that rings all the phones (whatever you have set Selector Code 8 to transfer to. I'm guessing that is one of the Calling Groups, Ext. 71 - 74)
Otherwise, you can go into the Partner Messaging, create mailbox 50 (for instance), but instead or pressing 1 to assign a mailbox to it, press 3 to assign it to an auto attendant. Specify the auto attendant, probably AA1, and then press 6 to create it without a physical phone. Then from the mailboxes, either instruct the caller to dial *850 to return to the auto attendant, or change the Personal Operator to extension 50.
If you are connected to the Partner Messaging via the LAN port, you can change the Personal Operators for the mailboxes for them, without having to log into each mailbox.