Sorry about the SVG manual, but there is no need to do anything that drastic just for an opening.

..cut out a part of the floor so that you can look from the top floor ...

Two simpler options spring to mind:
1) Draw the floor manually with the floor drawing tool, leaving a hole/cut-out where you want it.
2) Import one (or more) boxes as staircases. The boxes should be invisible, which you can get by using an invisible texture (like invisible.png). If you size and lift the box(es) to the ceiling below, they can cut out the hole you need. The boxes are possibly faster to change later, but may be in the way when you work on the ground floor (in which case you can make an intermediate level just under the ceiling just for the cut-out boxes).

Hope this helps. ok