How to do it: 

* Set one of your Sim's traits to Kleptomaniac (Sim must be female).
* Go to graveyard.
* Wait until Sim steals remains or tell sim to swipe something (remains must be male).
* On the second day since stealing grave and putting it in home, the ghost will appear
  at 11:00pm and disappear at 4:00am it will appear every other day.
* Socialize with ghost every night it appears until you are at best friend ghost will
  then begin appearing every night.
* get fertility reward.
* Confess attraction to ghost.
* ask to snuggle on bed, kiss.
* Try for baby every night until you start throwing up.
* Congrats! You're now having a ghost's baby!
  (It will be born as a ghost that you can control).