Hi
Yes, it's possible. Download the admin documentation from here
http://www.zimbra.com/community/documentation.html and on P41 it describes how to add the rbl amongst other things.
I did try this on the last release of Zimbra, I assume it's the same on this version, but if you have anything set in the admin gui (Global Settings/MTA tab then you must also add them again with zmprov - it only enters the options specified on the control line and will clear the other settings.