Answer from Benjamin:
The easiest way would be exporting them to *vcf format and then importing them on the Fairphone.
The Nokia 5230 supports Nokias PC Suite as well is iSync. I don’t know if you are able to export the contacts from the Phone directly to the SDCard, you can try to follow this guide: copy contacts.
If that does not work you can use your Windows PC, sync the PC with the Nokia and then export from Outlook or Windows Contacts to a vcf file.
If you have that vcf file, you copy this to your Fairphone, open the People App and press the Menu/Options button ( the left most button). Then choose Phone Storage or SD card to copy from.
This question is originally placed on the old forum.