Add new city borders from GML or SOSI files?

Hi, I am completely new here so excuse me if my question is ridiculous or in the wrong forum. Is it possible to add city borders from GML or SOSI files at all? If yes, is it a long or quick process? I am not familiar with these file types, but I would like to add the postal code borders in Norway to OSM if possible, and they are accessible in these formats.

Before looking at the technical aspects of the import, you have to find out whether the data is released under a license that is compatible with OSM’s ODbL license. You will also have to read the import guidelines.
Since you are new, I highly recommend that you contact the Norwegian community and find an experienced Norwegian mapper that can assist you with this project.

There are GML to shape file converters. The latter can be used to import data.