How to add the Grim Reaper into your Family/Household
Step 1: Turn testingcheats on by pressing Ctrl+Shift+C to show the cheats console and typing "testingcheats true" without the quotation marks.
Step 2: Make a sim die by starving them or by using an insta-kill mod.
Step 3: When the Grim Reaper shows up, Shift-Click him and click Add To Family.
From there you can control the Grim Reaper! His needs are always full, and you can't make them go down. You can make him socialize with other sims, and you can make him eat and drink stuff, and make him do any other things a sim can do.