I have just successfully exported my Apple Adressbook contact into zimbra and it was very easy. You need 1 utility called AddressBookExporter-2.1.2.dmg. This tool will export contacts and has built in templates. I used the yahoo template to export my contacts. I then logged into the web interface for the user needing the contact moved. Selected preferences and import/export option in preferences and selected import, selected the yahoo contacts type and clicked import. This procedure has worked for 2 macs that are being used and working great with z-push syncing to iphone.

I hope this helps someone.

Rueben