xilix,
3dmax is a rendering program.
Group all the first floor walls: select all the lines and planes of the walls. You might do this by taking a low-angle view and selecting all at once. Then choose the menu item edit / make group.
Then you can draw on top of the walls and the wall lines won't stick to the plane you draw.
But there is more to making a second story and ending up with walls free of lines inside and out, and a ceiling plane on the first level which is joined to the walls below. I was hoping someone more experienced would explain the most efficient process of getting to the second story.
Peter