How can I add a light switch to an existing lighting circuit?
Anything can be done, but in simple terms, NO you cannot switch one light that is already switched without using a hot and a separate switch circuit.  I will not explain in detail, cause I feel you really don't have the concept of how this can happen. Sorry if I sound rude.  If you do not have a basic understanding of electrical circuits, than I really recommend you get an electrician to help you. Electrical is not a DIY unless you really know what you are doing. It certainly can be done with some wire fishing, possible wall opening, and some refinishing work after. Be safe and get someone to look at your situation. We cannot tutor you enough here to assure a safe modification you want to do.