Tie switch into existing light

Yes you can add switches, how to do it is the question. Ceilings usually have some form of insulation, mine are preformed foam and the is little or no airspace. Exterior walls almost assuredly will have foam insulation. Fishing wires is a matter of skill and the right tools.

I googled "plastic self adhesive wire moldings, and got lots of different surface moldings and raceways and cord covers.

I've always tried to keep my mods hidden, keeping the walls looking original has always been a priority with me.