How can I add a light fixture to a three-way circuit by connecting to the switch?
Re-designate the wires between switches to be as follows:


Black = always hot
Red = switched hot to the light
White = neutral 
Green/bare = ground, as always

Find and obtain "smart switches" which support this wiring scheme.  They would need some wireless or power-line method to communicate with each other.  


One option that would work slick for this is neutral required motion sensors, since they  would not need to communicate with each other.  They need to be "neutral required" types because "don't need neutral" types rely on leaking currrent through the turned-off lamp, and this won't work at all if the other motion sensor turns the light on.  It also doesn't play nice with LED/CFLs.