Whoohoo! I found a way to add my incoming (by mail) .ics appointments to the N900 calendar! Not too efficient, but it beats typing by hand. This is what I do when I receive an event via mail:

1. open the email
2. 'save attachments' > save the event (anywhere, p.e. in documents)
3. exit mailapp and open a file browser, go to the folder in which the event is
4. open the file and watch the magic
5. choose in which calendar to add the event
6. hit save
7. see the saved-attachment-from-an-email transformed to a real event in your calendar

Hooray.
O.k., it is a bit of a workaround, but it ends my frustration of seeing the event being recognized in an email and not being able to put it in my calendar.

If anyone has a more efficient way, I'd still love to hear about it