Two switchboxes, power through light - how is this wired?
Here's how your circuit is connected.  



I've changed blue to white, I've labeled the screws on your 3-ways, and I've put red, black and yellow tape on a few wires to designate their function.  Red is switched-hot for lamps.  

Now, we have 3 separate circuits here in close proximity.  I added "Chinese Walls" to separate them. No wire can cross those, except ground. 

Now when you want to add additional lamps to a circuit that already switches one lamp, the best place to extend it is from the lamp.  But that's not what you're asking, is it?  You can extend anywhere you can get switched-hot for that light and the neutral for that light! Neutral from another circuit will not do!   


For instance lamp 1, you can see the left side of switch box 1 has the actually-red wire and the actually-white wire on the left side of the box.  That's an acceptable place to tap for more lamps with lamp 1. 
Lamp 2, the only place genuine neutral (not white-wire-used-for-another-purpose) and switched-hot (black marked red) coexist is in the light 2 box.  That is the only place. You cannot tap switch box 2 because there is no neutral whatsoever in switch box 2.  Even if switch 3 brought down neutral, you couldn't use it because it's from a different circuit.