Hi @Bogdan_Chipara,
Thanks for the suggestion.
Have you tried using 3D Sketch for this? You can convert a Basic Building to 3D Sketch, then sketch on its faces and use the Extrude Face tool to do what you're looking for. If you're trying to do this on a surrounding building, you can Add Floors to it first, then convert to 3D Sketch as illustrated in the documentation linked above.
3D Sketch buildings will behave a little bit differently than Basic Buildings, but they do have full 3D capabilities.
We plan to bring this 3D sketching technology deeper into the main Design mode, so eventually workflows like this may not require converting the building and entering a sub-mode. But for now, hopefully this can work for you.
Let us know if this helps!
Josh Goldstein
Senior Product Manager