Zimbra would just be the external MTA for a specific domain. I believe ISPConfig has the ability to work with an external MX, right?
So what needs to be done is create SOAP calls to the Zimbra installation to create domains/, mail users, etc. It can be done, I'm at this moment working on a commercial integration between a CP and Zimbra with a partner company. Sorry, that's all I can share at the moment.