The room mate option came with University. I can come up with two ways to add the sim to your active household:
First one is to type in "testingcheatsenabled true" in the cheat box and then shift click your room mate and pick the option "add to family" (I think it says). I'm not sure it works when the sim is already a room mate though.
If it does not work, you should get the option to "dismiss" when you click on your room mate. By doing that, the sim will not be your room mate anymore, so then you should get the "ask to move in" option again after some friendly interactions. If you do it this way, just remember to toggle room mate services off by calling "real estate and travel services" on your phone, and set number of room mates to zero.
I hope this helps!