For incandescent/halogen lights it should be pretty simply. You'd need a battery pack that puts out about 6 volts, 3 watts. Wire that to a switch and then to the light. IIRC, Bottle dynamo systems usually use the frame as half the circuit, so you'd either need to add another wire to the light, or wire your a battery terminal to the frame (connected to bare metal, not paint).

LEDs (which I doubt is what you are using) are a little bit more complicated. They care about the direction of the current, and so use a rectifier to convert the AC current from the dynamo to DC. Your battery will put out DC, so I think it would just be a matter of finding out which connection should be which.

Check out dealextreme.com for batteries and parts. Also, the electronics & lighting forum would be a good place to ask any more detailed questions.