One of our clients is going back to Exchange

but they want to keep the Zimbr MTA setup as a gateway. I have been able to get the gateway aspect configured manually and tested that it works. However I know that
transport_maps parameter is rewritten fresh upon each restart. I need to add another transport_map entry to this line. How do I accomplish this within Zimbra? I looked at zmmta.cf and I have a feeling the answer lies in there but I have no clue how that file works.
thanks
Doug