How to add vertices to certain area

So I am making a map for my game and I gave it a texture, and I want every building on there to be 3D. And I do not want to add new objects to the original. I was wondering if there was a way to add vertices to JUST a specific area so i could just extrude up? I tried using the knife tool but it was way too buggy. Any other way? Basically just add a face of a certain size to a certain spot on the original face. Also I would have multiple.

Basically just add a face of a certain size to a certain spot on the original face

K = knife
I = inset
W / Subdivide

Also I would have multiple.
Multiple what ?