It sounds like you want to cut a cube sized hole in a bigger cube. Your ideas of creating a cube area in the middle sounds perfect and then delete the faces and finally make new faces going “into” the cube for the sides of the window. The subdivide multi with 2 as suggested by Hobo Joe does sound like the best way to go.
Just for completeness another possibility would be the creation of a “Window” sized cube and use that as a template to cut the hole. You would use the “boolean” functions and select “Difference”. So you would basically be subtracting the Window sized cube from the wall. I am an old POVRay guy and am used to this kind of technique but reading the forum lots of folks say that the boolean functions are not that great in Blender. You will certainly end up with a log of extra vertices but they should all be on the same planes. Personally, I’d go with your original idea.