That's a great question!
I bet you would have to do a mix of adding in set knowledge that they're living with an NPC, Set a Routine where they sleep in that NPC's house; on the floor until you can add a bed, and possibly Set Relationship to that NPC to at least friends if not Important, though I have no idea if that matters, it would make sense as a long term living situation if they were friends at the least.
On a similar beat, what is the location/name or code for Gareth's shack?
Working on a new scenario that was a fun concept before of the player being a much needed smith, but of course this would require another character to rely on as a partner to work up the engine and "build" in text. So I figure another character pining to get their smith on at the shack may do the trick. ...Or I'd have to add the behavior after it's been opened, unsure.
Regardless, since this could potentially let us set up an NPCs behavior pattern, I could get a reliable 'worker' in there rather than brute forcing NPC's or Mirel. At least I think/hope so.
I think it's "Abandoned Shed", but I don't know if buying it changes the name to something like "Shed", or "%PLAYER_NAME%'s Shed" since it's no longer abandoned once bought.
I think you can build a new floor, unconnected to the shed, on your land, and you should get a system pop up to name the new building, and it should have the name of the area/shed as default. Otherwise, I don't know.