You can use the Boolean modifier.
First create a cube, then scale it so that it intersects with where you want the door to be.
The hole will be cut where the red line is. Next, click your building and add a boolean modifier. Set it to difference instead of intersect, then set the object to the cube you just created.
Now click apply, and delete the cube used to cut the door. You should end up with something like this:
If you enter into edit mode, the extra faces created should automatically be selected. If you want, you can delete the extra faces and get something like this:
To do what you would want, you could select the vertices around the first doorway, press F to make it a face, then press P and select by selection.
This has created a plane that has the exact dimensions of your door. It is a separate object. Press Tab to go into object mode, click the door plane, and press Tab again, to enter edit mode. Then select everything if it is not already selected, then press E for extrude.
You can then click once it is thick enough, then go back to Object Mode. Move the cube so that it is intersecting with where the door should be projected onto. Then do the same thing I mentioned above with Boolean, only use the cube you created with the door instead of a new cube.
Tada, two identical doorways!