Which file(s) do I have to override for giving my hired adventurers...

[Seriphos]

..background/cultures that are companion-only? For example, Serafen has culture Principi sen Patrena and both Eder/Xoti have the background Farmer. Is there a way to use these companion-only cultures/background on your hired adventurers through modding?

[house2fly]

There's a mod for backgrounds: https://www.nexusmods.com/pillarsofeternity2/mods/232 it makes all backgrounds available for all cultures during character creation

 

Cultures, I'm not sure about. It looks like you should be able to search Characters for the ID# of the cultures you want, then add those cultures to "CharacterCreationValidCulturesIDs" in Global.