There’s a PR right now for a new NPC, which shows the typical level of effort needed to create one: https://github.com/CleverRaven/Cataclysm-DDA/pull/29529. It really isn’t that much: add some text to a couple of text files.
I’m not sure what “procedurally generated backgrounds” means in this context. If you add a new faction to the game, you can do what you want, within reason. If you want all your NPCs to get randomly assigned backstories, you can do that. If you want them to have only certain backstories, that’s also an option.