I do know if you put a gravestone in the graveyard, you can still pick it up anytime you want, of course it has to be the sim that put the tombstone there in the first place, I tried it and it did work. I was doing an experiment on ambrosia and went to the graveyard to bring Jon Lesson back, but you cannot be sure of which ghost is going to eat ambrosia, but for the most part, ghosts in the graveyard are NPC and if you bring one back, they get in a car and go across town and then disappear, so if you want to play someone you brought back, you need to talk to them first, so when they go away, you can call them on the phone. My sister brought back Eric Darling for Crumplebottom, he got in the car and went away. Later he showed up as her coworker in the diner. It is pretty cool to bring someone back though, especially if it was a younger sim that died of drowning or starvation.