I stored my addressbook with Nokia's software, and it stores it as an NBU file. I used NBU Explorer found below:

http://sourceforge.net/projects/nbuexplorer/

With that software, I converted it to another file ... sorry cannot remember but it should be self explanatory in the format it can convert to. This then was imported into my n900 through the phone. At the time, the N900 wasn't supported by Nokia's software suite, maybe it is now easier to import.