Trying to do ghost stuff

I don't play with ghosts much but I think you're trying to make him playable. I did this once with the Philosopher's stone. I don't know which Expansion it comes with but think it's from Ambitions. If you use it to bind a ghost the bound ghost joins the household. There are two ways to get it. You can use a lot of Lifetime Happiness points to buy it as a reward. I've never done this. The way I get it is to master a skill. I think it's inventing. This will lead to a series of opportunities to deliver things. I think there were a series of 3 opportunities and when they are all complete you will find the Philosopher's Stone in your family inventory. You must have room in your household to add at least one more Sim to successfully bind a ghost.

EDIT: See @Auroraskies post below for correct info