Also, even when they do come out, you won't be able to ask them to move in. You'll still need to wait for the Oh My Ghost opportunity to get them playable.
The other option (which I think is easier) is to have a ghost baby. If you have Generations, you can have a sim use the chemistry set until they develop the ghost potion. That'll only make them a ghost for a few hours, but if they try for baby during that time, the baby may be a ghost. I think it's a 50% chance if one of the woohooing sims has taken the potion, and 100% chance if both of them have.
My favourite way to get a ghost baby needs the Ambitions expansion pack. You just need to get a female sim into the Ghost Hunter career, and work up a few ranks until she starts getting the jobs which have actual sim-like ghosts roaming around. When they do those jobs, there are special social interactions - I forget the exact names, but they're along the lines of reassuring the ghost about their loved ones and reminding them that they lived a good life and stuff. Those socials raise the relationship bar in huge leaps -you can max out the relationship bar with a few of them - and then all you need to do is a few romantic actions until you get the option to Try for Baby. And then you hope that whoever owns the house you're in has somewhere suitable for Woohoo and that they're not asleep in the double bed... The Inappropriate In A Good Way LTR helps here. Once you're done, you can zap the father or convince him to move on (I always feel bad about zapping them after...). There's a 50% chance that the baby will be a ghost, and if it is, it'll be the same colour as it's father. Which is the big benefit of this method - you can pick which colour ghost you want, by waiting for a male sim ghost of the right colour to turn up!
