How Can You Add a Sim to the Same Dorm, Yet Not Have to Control That Sim?

I don't know a sure way to do it, maybe someone else will. But, I did do it "on accident" once. I changed my mind about having a second Sim living in the dorm with my main Sim. I separated him from my household and moved him to other households. Instead of just disappearing into townie limbo though, he continued to live in the dorm I was in.