Zimbra should run on it's own piece of hardware/VM. Don't mess with it. I know, cause I'm talking directly to the people of the Zimbra company. You can combine the two by making use of the external mail server option of ISPConfig.
Then what needs to be done is to write a SOAP extension for ISPConfig to talk against the Zimbra API's to create and manage users.
I can only tell at this moment in time, I'm very keen to combine ISPConfig and Zimbra.