Patch 1.38 has updated a ghost’s CAS options by adding the ability to choose death types for Sims, and keep them. The easiest way to make it happen according to Tom and Annabelle, is to Edit the - already ghostly - Sim in CAS, Choose a death type, Save to the Bin and exit. Once you go into Edit Town to “Create a New Household”, you will be able to select the ghost and drop it into your household.
I tested this to see if it works, and it sure does! It is note worthy that you remember which Sim you customized as a ghost because it will show up in your CAS Sim Bin as a regular, non-ghostly, Sim. If you Edit a Ghost in CAS, make changes and then exit, the Sim will not change appearances. The steps above must be followed to make this happen.
All credit goes out to swdknight who first discovered this.