I hope you don't mean the "Smart Birthdays" calendar of the N900. This is based on the birthdays you give to your CONTACTS, which are then exposed as a sort-of virtual calendar.

I don't know what nbuexplorer is.

From the calendar app you can go to settings - edit calendars - and then for each one click on it (e.g. "Birthdays") and then "EXPORT".

This will export the calendar as an ICS file, which google (and other programs) can read easily.

Obviously if your N900 is dead already you'd need to do this by hand.