I observed this today when playing an ex-challenge file. I had read a method of controlling Grim (doesn't work, I must edit it out of the Sims Wiki), and I found a way after doing that.
With Generations, or possibly patch 1.22, minimum, you can control Grim.
First you need to enable TestingCheats. Then you need to enable moviemaker cheats (the code is "moviemakercheatsenabled true").
Shift+Click on the mailbox then pick Spawn/Force NPC>Grim Reaper.
Now Shift+Right Click on Grim (when he appears), and voila! I noticed that you qill get to Grim's "family" as well - you will control several random townies. To return to your family, shift+Right Click on your sim.
Grim will vanish, and I'm trying to circumvent that without using mods.
I just found this to be a nifty thing to see. He does change LTW and skill points (He had level 5 and 6 Logic when I tried!) each time you spawn him, I believe.