Hi - after much head scratching... and a fair degree of swearing(!) I have successfully uploaded a data file from Outlook into Zimbra.
Its surprisingly straight forward when you know how(!). Here is how I did it:
1. Mail - Make a copy of the PST you would like to import.
Set up an IMAP connection to your Zimbra account in Outlook
Import the copied PST to the IMAP account
2. Calendars - open the calendar you would like to import into Zimbra using Outlook. With the calendar open, go to file/save as, ensure .ics is selected as the file type, click 'More Options' and ensure that the date range is set to 'Whole Calendar', detail is set to 'Full Details' and that the two boxes under 'Advanced' are checked.
Open your Zimbra web client. Go to 'Preferences' and 'Import/Export'. Import your .ics file
3. Contacts - open the contacts folder you would like to import into Zimbra using Outlook.
Open the 'File' menu and choose 'Import/Export', choose to export the file as 'Comma Seperated Values (Windows)'
Open your Zimbra web client. Go to 'Preferences' and 'Import/Export'. Import your .csv file
Hope this helps!