ThreePhaseEel's comment is key. Most light switches I've seen are set up the same as in this instructional PDF. The relevant portion from that PDF is in this illustration:
Likely, this is exactly how the switch with one light is wired. The switch with two lights starts like this and then has another run to the 2nd light box. The first light in the circuit will have 3 conductors in the wire nuts of that lamp. If you can make a run of romex NM from the single light circuit to one of the lamps of the 2 light circuit, you'll be ok.
Disconnect and cap off the feed to that other circuit as well as the wires to the unused switch. Then you can add the light to the desired circuit.
I started an illustration but that'll take longer to draw than to do the connections.