Sam, I actually thought of that. But I realized that they'd need to go to school, which would be a problem.
Anyways, I made my founder have a baby with the basement dude (Impossible... took them three tries.)
I kept all the other sims (the founder's husband, the basement dude's girlfriend, and the adopted child of the upstairs couple) out of the house for a good amount of time by sending them to tour the military base and then the science facility, but they ended up at home just before the illegitimate couple finished trying (thank god she got pregnant that time).
So everyone's relationship was seriously messed up at the time because everyone found the two cheating, and even an infinite amount of apologies didn't seem to be working, and everyone's needs kept failing because of all the social work being done. Anyways, everyone ended up on good terms with everyone else.
So my founder got pregnant and ended up having a baby, who I named Sarah, and, the moment she took Sarah home, I decided that that was the time. The basement dude got a glimpse of Sarah and ran to the basement for the last time. He went to sleep with his girlfriend and I deleted their door.
No problems have thus far been incurred, although no one's quite needed to eat yet. I built them a kitchen and a bathroom. Apparently, there are no negative moodlets to be had for being locked in a basement.
The only problem that I have thus far is that the basement dude's girlfriend was an NPC, and she's almost an elder at this point. Fortunately, I have it set on epic lifespan, so almost means about 35 days left, but my founder hasn't even started fishing for deathfish, and I suck at fishing (as a player), so I don't even know if I'll be able to get too many deathfish.