How can I add additional lighting fixtures to an existing 3-way circuit?
Yes.  You'll connect the wires feeding to the next fixture, to the wires that feed the current light fixture. 

Basically, you'll start with this...



Then you'll add the additional lights like this...



NOTES: 


I've excluded the grounding conductors to simplify the diagram, make sure you properly ground all the fixtures.
The black wire coming from the "3-way wiring" bubble, might actually be white, red, or some other color.