Well, I don't know how to make Grim playable in the way you're asking about, but I do have a complicated way of the next best thing. This is assuming you have custom content all set up and ready to go.
You'll need this
"Chaos Mod Painting" here from Modthesims.
You may or may not need the
"Grim Reaper No Leaving" mod. I'm going by the method my boyfriend used, and he has it, so you may need it. I have no idea, really.
I'd also recommend you back up your game JUST in case. But that's true of anything.
Once you have that installed and good to go, follow these steps.
1. Get a sim on your lot that you aren't attached to. (I think my boyfriend just killed the paper boy when he came to deliver the mail...LOL. For example's sake I'll just use the paperboy here.)
2. Click the Chaos mod painting, go to "Voodoo" and then "KILL". Kill the poor paper boy however you feel like it.
3. Good ol' Grim will show up and take poor Billy the Paperboy(or whatever his name is) away. Sad day.
4. Before Grim leaves, click the ChaosMod painting again, and select Voodoo...KILL. Now, when it has you select a Sim, click on Grim's face and give him a taste of his own medicine!
5. Grim is going to fall over and die, and it's going to be kinda glitchy looking. He'll be taking himself away, after all, so the game is confused.
6. Grim's tombstone will appear. Yay. Dead Grim. Now, wait until night time when Grim shows up as a ghost. Spooky.
7. Go to the ChaosMod painting, and somewhere under Mortality there will be an option like "DeGhostify" or something like that. Can't miss it. Click that, and select Grim!
Tadaaa.
You should be able to play him just fine now. His traits might be different than those of the original reaper, as my boyfriend reported traits that were different than whatever I got talking to Not-Dead-Grim...but you can always fix that with TestingCheatsEnabled, shift click, modify traits and have Grim be however you want personality wise.
Hope this helps.
You're braver than you believe, and stronger than you seem, and smarter than you think.
-Christopher Robin to Pooh