I haven't needed to export calendar data, but I've been able to export contacts from Entourage and import them into Zimbra easily. Here's what I did:
1. In Entourage Preferences, General Preferences -> Sync Services - check "Synchronize contacts with Address Book and .Mac" Click OK, then select "Replace Sync Services items with Entourage items".
2. Open the OS X Address Book, select all contacts and then File -> Export -> Export vCard.
3. Upload the vCard data to the Zimbra server using the command from the wiki:
User Migration - Zimbra :: Wiki
curl -u ZIMBRAUSERNAME:PASSWORD --data-binary @/PATH/TO/vCards.vcf
https://zimbra.server.com/zimbra/hom...ntacts?fmt=vcf
It seems to work very well as far as not munging data.
Best,
Martin