Vako,

What you need is a combination of a door frame and a service hatch, but than invisible. This model isn't provided, but you can make it yourself with a bit of effort.

1 - Combine 3 very thin boxes into a door frame (2 vertical, one horizontal at the top)
2 - Export to OBJ format
3 - Using a text editor, add "d 0" (see this thread) to make it invisible
4 - Now import it as furniture, and make sure you check the check box "door/window"

If you insert this model, it will function as a door frame, but because there's no floor part, the floors of two rooms will connect seamlessly.

Maybe (very maybe) I will make the model after the weekend. Now it's time for bubbles, lots of bubbles.

Hans