There is/was
a lot of discussion on having an external XMPP (Jabber) service work with Zimbra. We
might (as in possibly, but not sure) see something about this in ZCS 7 or later.
On the other hand, you already have the capability to have external IM clients connect to your existing Zimbra environment. I use Pidgin, but you're free to choose your own. You just need to point them to your mailstore server, since it doesn't go through the proxy (I think there's an RFE about that). The IM users will have to be Zimbra users, that is, they will have a full account on your server.
More info:
Pidgin and Zimbra - Zimbra :: Wiki Adium and Zimbra - Zimbra :: Wiki