Agree with uxbod, but I think I understand what you're getting at. I'll give it a shot.
We recently did this with a domain that was being migrated to our Zimbra servers. We kept the same domain between both the old mail system and Zimbra. First we created the domain in the Zimbra Admin and executed the following:
On the old system, they had to setup forwards pointing to our MTA for each account as it was migrated. I pulled this from Split Domain Configuration.
$ zmprov md <orginal domain> zimbraMailCatchAllAddress @<orginal domain>
$ zmprov md <orginal domain> zimbraMailCatchAllForwardingAddress @<orginal domain>
$ zmprov md <orginal domain> zimbraMailTransport smtp:<old MTA>
A word of warning though, be careful with the 'zmprov mcf' commands listed lower on the page, as these modify global config. settings. These will redirect all outgoing mail to the old mail system. If this is the only domain on the Zimbra server, that's cool. If it's not, this will cause problems.
Hope this helps!