I'm new here and would like to help in regards to this topic. Please let me know would this link below be more helpful here? There are official adm codes, relations in between regions and municipalities, listed both in Serbian Cyrillic (@rs) and English (@en).
The regions seem to have different codes then the ones we are using. Do you want to prepare a list with the information to be added? (geonameid, name in Serbian). It would facilitate the update a lot.