Adding a new switch with an independent light in an existing circuit
Based on the input by the contributors, I was able to make an informed decision. I passed a new cable down through the wall to the main switch box at the bottom of the stairs. I wired in the new wire and passed the wire across to the pantry room. I removed the wire from the fixture and tied those together in a junction box. I passed the new cable down to the new box and passed a new wire from the box to the existing electrical box. The fixture box is plastic and doesn't have a ground screw so I just capped it off with a wire nut. I wired the light and the switch without any issue.