twouters is right iam pretty sure you can import a .csv file
Now I know there are threads on how to import sms messages on this forum, but I get errors attempting it. So not to try to attempt to duplicate any current threads but I would like either a detailed thread with instructions how to import sms messages
With nokia pc suite you can export all your messages to a .csv file. You'll probably be able to import them into the n900 with it too.