I don't see a problem with this. You can create the beam using a box and elevate it so it's flush with the top of the wall.

I assume that the beam is on the outside of the wall and that you want the roof to extend over the beam. It depends on what kind of roof you want but the easiest way is to use the Generate roof plugin to create a roof.

If you don't have that plugin installed, you should install the Plugin Manager (direct download) and use that to install and keep up-to-date other plugins. Read more about it here .