I've managed to make a bar counter for a Bar I'm designing for a story - (out of a wall), but I want to do further customization to it but apparently I cannot 'stack' walls at all.

I wanted to do a box wall, then a flat 'counter' board on top of that.

I dunno if there is anything I can do to further customize that wall. Any Wall object HAS to start from the bottom, apparently.

Help? Or do I just export the Wall to Blender, and try to make a counter OBJ file?