There are a couple things you want to check out...
You can use an Internet Subscribers for this. With Internet Subscribers, the destination SMTP address can easily be changed on the fly.
You should also check out the Migrate Subscriber Data tool:
http://www.ciscounitytools.com/App_MigrateSubscriberData.htm
Keep in mind that each Unity subscriber can be associated with one and only one Exchange mailbox.
Hope these help,
Keith