Now this place is just begging to converted into a criminal cesspool, and I aim to do just that!
What's going to happen is the building you make is going to serve as the overground "shell" for the actual rabbit hole. Once you've made a building, all that is required is to make a basement, and put the rabbit hole down there!
So create a stairwell down, and give yourself a nice big block of basement. Input "moveobjects on" in the cheat menu, and place the rabbit hole so that the door is reachable by sims. The entire rabbit hole does not need to be in the basement, just the door.
Here you can see the rabbit hole being operated by one of my more nefarious sims. Just another day's work for the dastardly!
Edit: seems I was beaten to the punch. Oh well, maybe the pictures will help clear any gaps.