Adopting NPC factions is indeed possible - see Marlinist factions being adopted by a Squadron (with FDev's apparent blessing).
However not all PMFs accept that this situation - see invasion and subsequent harrassment of Marlinist systems by nearby PMF based on "No PMF, no claim" argument.
However not all PMFs accept that this situation - see invasion and subsequent harrassment of Marlinist systems by nearby PMF based on "No PMF, no claim" argument.