Can I add a three way switch to a light circuit when I only have access to light switch and it just cuts the power to the switch?
Yes you can. You need to replace the current switch with a three way. 

Run the new three wire cable between the old switch box and the new switch box.

The wire that carries current to the old switch is attached to the common terminal of the replacement three way.

The wire that brings current to the light in the existing switch box is connected to one of the wires to the second remote three way switch. In the new switch box, this will be connected to the new switch's common terminal.

The other two wires are the travelers between the replacement switch and the new second switch.

NOTE: Be sure to mark the white wire in the new cable with black tape or marker on both ends. That wire will be hot.




Old Wiring


New Wiring