is there a way to modify the GAL for a domain yet?

We have a multi-domain zimbra install and are looking for a way to add certain external email addresses to specific domains, my only search yielded someone suggesting setting up a distribution list that goes to the external domain. not only is that not really going to work in my situation, it would only create more confusion among my users.

so is there anyway to manually add an address to a specific domains GAL?

We are currently running 4.5.6