Let's just say, I have my pretty cool Sheriff's Office station. And there are holding cells, which are empty. Is there an option which could allow me to spawn some NPCs in those cells, and keep them there forever, which means they'll be there everytime I go on game?

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This can be achieved via menyoo, (albeit with an asterisk attached).

 

You can use menyoo's spooner to spawn ncps in the cell, by default they'll stay in place (ie won't flee from gunshots) and can have animations set, such as sitting or sleeping if you want them on the bed.

 

You can then save them as a file, which is where the asterisk comes in. While they will persist during the session, on a new load they'll be gone, but can be reloaded via loading the file they're a part of, returning them to being permanent for the duration of the session.