Adding Light to Existing 3-Way Switch Configuration
The right hand side has the 2 travelers and a return this was the old way of wiring a 3 way , the left box the neutral goes to the light. The only legal way would be to tag the existing light or bring the power from the left hand switch. It would not be code compliant to tap another neutral in a separate cable even if on the same circuit. This is based on your drawing.