Wait for somebody else to say I'm wrong
If all you want to do is test out the web interface instead of installing a Zimbra server and then trying to find some hack to get it to ignore most of it's functionality you should probably look into
Zimbra Desktop as that is basically the Zimbra web interface as a wrapper around a custom mail client, which will allow you to have the Zimbra web interface while using IMAP to your existing server.
The problem with this is that just like adding an external account, this is a per user thing.