Can I add a second light fixture to a 3 way switch?
If those three wires are the only ones in the junction box, then you're out of luck. The red wire and the black wire are travelers going to the other switch. The white wire appears to be connected to the black, common, screw so it's going to be a switched hot going to your light at the bottom of the stairs. There is no always hot or a neutral so adding a new light is a no go. Can you run wire from your existing light to the additional location?