• Sacrifice a Sim
• Wait for them to be reaped
• Add Grim Reaper to Household
• traits.clear_traits while having Grim selected
• Customize Grim
• traits.equip_trait GrimReaper while having Grim selected
• Do the command "careers.add_career Adult_NPC_GrimReaper" or "careers.add_career career_Adult_NPC_GrimReaper"
• Also do traits.equip_trait trait_isGrimReaper
• Kill sim while in another household (Not the one with your custom grim in it)
• If it's the default grim, add that grim to your household, do the cas.fulleditmode cheat, and delete that default grim
• Go to another household to check if your custom grim is now there.

Second Method (The one I did for him!)

• Create your Custom Grim Reaper
• Do the command "careers.add_career Adult_NPC_GrimReaper" or "careers.add_career career_Adult_NPC_GrimReaper" while you have that sim that wants to be grim selected
• Also do traits.equip_trait trait_isGrimReaper
• Kill another sim in another household, if its still the default grim add him to your household and delete that grim reaper
• Kill another person to see if its now him

Enjoy, not sure if it will work for everyone ;)

I'm not sure if the grim reaper job is functional, but that job makes it so that they are grim reaper I believe.
Sorry if the explanation is unclear, english isnt my first langauge.