Okay, time to revisit this.
DirectAdmin's email managment is highly integrated.
Yes you can have multiple mailservers on one running DirectAdmin installation, but the second one isn't going to do much.
Why?
Because DirectAdmin creates mailboxes, accepts email, and delivers mail, based on specific integration with exim, and DirectAdmin
knows to build specific file structures which are used by the current exim.conf file. It's doubtful that you'll be able to get Zimbra to read the files DirectAdmin creates and uses, and it's doubtful you'll be able to get DirectAdmin to read the files Zimbra creates and uses.
Currently exim is bound to port 25 and 587, and Dovecot (in updated installations) is bound to port 110 and 143.
So where are you going to bind Zimbra? For example, bind Zimbra to port 2525. Now, how are you going to tell the world that for some domains email should be sent to port 2525 instead of port 25? Maybe you're some kind of networking guru who can figure it out; go for it.
Or perhaps you'll decide to shut down the exim and dovecot services on your DA system. Go for it, but then NOTHING in DirectAdmin that's in any way email specific, not even creating email addresses, is going to work. So at the very least you'll have to create an entire new front-end (skin, scripts, etc.) for every email function in DirectAdmin.
Go for it!
Jeff