I haven't used any cheats yet, but I've had two ghosts move in with me.
First, I built up my guitar skill and just decided to go play for the ghosts in the graveyard. After a few hours of playing (Only after dark, mind you.) TONS of ghosts wake up and wander around and they are easy to talk to. I don't know if the guitar really matters, I'm just mentioning it because it's how I did it.
At some point in the game I got a random opportunity asking me to take part in reincarnation research. I went to the science lab and nothing much happened, but I got the LTR points. Again, I don't know if this matters, but it's what gave me the idea that ghosts can reincarnate or reanimate.
After becoming friends with a ghost, I picked up his headstone and put it in my inventory. I brought it to the science lab and it offered me a choice to attempt to reincarnate the ghost. He didn't become a true Sim, but he became a ghostly playable character, added to my family.
Later on, I sent him back to the grave (An option on his headstone), and repeated the "Pickup headstone - bring it to the science lab - reanimate ghost" behaviors and it worked again.
So I THEEENK If you just steal someone's Headstone and take it to the science lab, you probably will get the offer to reanimate them. Someone else try it!
Oh, also, if you don't want to PLAY the ghost, you can place their headstone on your property and it will wander your property at least a few nights a week, where you can interact with it as if it were a townie.