The social debug dialog just grabs random Sims and strings. I think in this case I was trying to get a specific Sim (who was not a cat) to accept becoming a roommate.

Yeah, you don't get a choice if a cat wants to be your roommate. XD

Now I want a hack where the cat distribution system makes cats randomly show up at sims' doorsteps and want to move in. XD

At one of the houses I downloaded, stray cats kept getting stuck on the porch, so my Sim who lived there basically had to adopt them or they'd starve. I don't use that house anymore but I still play that if a stray cat keeps showing up on the lot, I take it that it's insisting on being adopted, so I use the Sim Blender to move it in.