Hm, are you using Mac only?
With Windows, you need to install Nokia Suite (a Version that supports N9) and then just plug your N9 to your PC via USB and press Sync, then all your calendar entries from your N9 are in your Outlook calendar (assuming Outlook is installed).
After that, you can plug your N900 and sync again, so all entries will be on N900.

Maybe it also works also without Outlook, meaning only with Nokia Suite, but i never tested it...