The power supply is what controls which camera is on at any given time. Each camera needs its own power supply, which an individual address.
You can certainly extend the power supply wires, but I really don't recommend trying to combine the wiring from the power supplies together. The power supply tuns the camera on and off according to the appropriate X10 commands, so you'll run into trouble if more than one camera is trying to feed video over the same line at the same time.
You would need a pair of wires (possibly two pairs, depending on the amount of current the camera requires) for each camera. I don't see a way to put all that, along with the video, in a single CAT5 cable.
Depending on the model of cameras you have, it looks like the power supply had one "ground" wire, and two "positive" wires. One was switched on and off with the X10 signal (turning the transmitter on and off), and the other was always on (keeping the camera optics powered up, to reduce "Warm-up" time. I think some of the cameras didn't have that second line, and turned the entire camera on and off with only a switched power line.