We don't yet have a consistent way of adding new postal codes. We would like to integrate the postal codes with the main database. For some countries we have started to add them as alternate names with the pseudo language code 'post'. France is more or less complete.
You can add postal codes as alternate names to a place name, though they will not immediately appear in the postal code listing.
You can also send me a file and I insert the codes directly into the database.