Externalizing a board

Hey RIOT users! I need your help: Is it still / somehow possible to externalize a board? There is the external module guide, but I didn't manage to quite get it to work. We have a board that is not available for buying, but open source, therefore I think integrating it into RIOT might not be quite useful? Or should I still make a PR for this?

Furthermore, can I integrate a tool externally as well?

Best Regards, Robert