migrate from single to multi server setup

[Cringer]

I currently have Zimbra 7.1.0 NE running on a single server. I have a Postfix server running in my DMZ as a front-end server.
Now that disclaimers have been fixed, I would like to change the front-end to Zimbra and purely an MTA, but keep the current Zimbra s it is.
We have various system setup to send out email automaticaly, and these all point to my current internal Zimbra.
The question is: What do I need to do to my current zimbra to make it talk to a new front-end, and what firewall changes will I need to make.
Plus can I have both an internal and external MTA?
All the guides presume you are starting from scratch, and a lot of us, usually like to walk before we run.
-Si-