The append polygon tool is used to create polygon faces to fill empty holes by clicking on the edges of the empty spaces, but still instead of going through that trouble, I think it would be easier if you use extrude faces instead.

just make a cube, cut it in thirds using the cut faces tool and then extrude those faces to create that doorway. All you do is just move the middle side face inwards and then then extrude the outer left and right border faces until you create the doorway.

i hope that helps. good luck on your work.