How do I add a light fixture to an existing 3 way circuit when the existing one is powered directly?
I cannot see it being able to be done. Theres just not enough wires. Basically whats missing is after you get your switched power to your light you need a path back to the main (source) neutral. Or any neutral really. You do have white wires in the 3 way switch boxed but they are acting as travellers and not neutrals. Sorry for your bum luck. I hate being short wires!
If you could pull a 3 wire to replace the 2 wire between the source power light fixture and the 3 way switch box on the right, than you could then feed a light fixture off that 3 way. I am not sure if thats easier for you...