Run line cords from extension ports on the ACS or its expansion cards to the ports on the Partner Mail. Put the extensions in Hunt Group 7 and administer everything else just as if you had Partner Messaging, Partner Mail VS, or Partner Voice Messaging.
You may have one gotcha. When ACS first came out, there might be a situation where the Partner Mail will receive a false ring signal, try to answer, not receive mode codes, and transfer to the Operator. The Call Answer Service Operator will ring once, which will annoy the users. Avaya tech support used to be able to dial in through the modem attached to the Partner Mail and apply a patch, but that's probably not possible at all any more. If you find that problem, using an old 206 expansion module may avoid it.