National place identifiers

Polish national statistical office publishes official list of Polish administrative units (like regions, counties, cities and even streets) – the TERYT registry. This data is public and freely available. It could be useful for quality assurance, setting the ‘is_in:*’ attributes and even for drafting administrative boundaries (as the official maps are copyrighted). If that data is to be imported/merged to OSM in any way then we need a tag to store TERYT identifiers of places. These are not postal codes or addresses.

What would be the best way to tag that information?

Reuse any of existing tags? Define new, like: teryt:terc10=, teryt:simc10= (TERC10 and SIMC10 are identifiers for places and higher administrative units)?
Are there some guidelines for adding/using custom tags for local data?

Ok, I have found the answer: