Here is my situation. I have a light that is plugged into a wall socket. I have put a switch on the light cord. Can I add another switch to this so that I can have access to a switch at either end of the shed.

You can but you will need to convert to 3 way switches and 3 conductor wire. That way, you can turn the light on or off from either end. If you do it as you hope too, you will turn off the power at the far end and be in the dark. Sorry.