TBA, the idea was acknowledged by Finchy around v.0.1.3, and the idea has been getting more and more hyped. Finchy wants to incorporate the idea, but...
A: Who gets added is kinda community based. Don't want to add someone who no one asked and say they are your roommate.
B: Finchy needs a "sprite map" made for the roommate, since none existed before. Also a place to place the sprite in the limited area of the base. (An idea I had was simply say that the guest broke a hole in a wall and revealed a sealed hallway with rooms. That way the player could have possibly 4-8 guests without any room being overcrowded.)
If for some reason you are able to make sprites for RPG maker of any girl, that probably would speed up the idea and add it in a later version. (Ideally, using the tread of "Best Girl" for an idea on who to make a sprite for.)