You can do that with cheats enabled too.
Just shift click on the npc and choose "modify in cas", if you wanna do in depth editing enable the fulledit cheat (cas.fulleditmode) so you can change everything about them like you can when making a new sim.
No mods needed.
I did this for all the sages in RoM, I am on console so no mods available and it worked perfectly, they stay unplayed and not in world.
The only thing it doesnt change is what they are called in the households overview, but that is the easiest to change.
