To anyone who is wondering, I was able to do this and have a child who is grim's. The steps to add Grim to family were offered to me by another user Skipping steps one through four because it involved using a mod not needed so long as Grim is at your lot because of a Sim death. Reapy will show up to lay the smack down on your victim, shift click him and choose 'Add To Family' Step 6: You will realise at this point that you can't woohoo with him or kiss him, but we can fix that.
Open your cheat console and type cas. Now he's a normal Sim and you can Try For Baby with him and even marry him if you like. Now, I was reluctant to do all of this, as I, one didn't want to mess up my Grim and have him not be able to reap souls anymore, and two, was sad that he would now be considered a "normal sim" after doing the deed, when really I only wanted to do this once or enough to get pregnant , I was playing my Olive Specter storyline and needed a baby Nervous on the the way for any of you who don't know Olive Specter is a premade sim in The Sims 2, who is believed to be a black widow, and her son is the child of the Grim reaper, as she has a memory for woohooing the reaper just before having her son.
Grim is good at cleaning up after morning sickness, lol Feeling the belly, kind of creepy, well all of this is really haha About to give birth: Grim is Nervous's daddy!! Now, after doing all those steps, Grim is a "Normal Sim", you can change his clothes in CAS, although I wouldn't recommend changing his everyday wear.
You can give him traits and an aspiration. Changing him to an ADULT makes him a normal sim and gives him the ability to have romantic interactions. BUT he will not have the creepy grim voice anymore, and he will not float or have the smoke when he floats. I was also unsure of what would happen if a sim died, would he show up? To fix this issue: I was browsing some videos and found a comment that someone posted giving this cheat: I didn't see it add any "trait" but all of a sudden he was floating, had the smoke, and the deep grim voice again.
Also, he was again, unable to do romantic interactions, he was back to normal! I then attempted to drown a sim on Olive's lot, and sure enough, Grim appeared, as normal, the same Grim, and he reaped their souls, and disappeared per normal. I was very pleased this all worked out, I have not grown young Nervous into a child yet, so I don't know what he looks like, he just looks like a normal baby right now.
I wanted to share all of this with everyone because I know there have been some questions, and I know like me I am sure that there was uncertainty as to what the repercussions of this would be.