A power inserter could probably not be done. The better solution would be as McNasty suggests, but maybe just replacing the last ?? feet of RG-6 with the "siamese" cable that is available with RG-59 and a pair of 20 gauge wire in the same outer sheath.

I don't think you could make a power inserter because the sync pulse of the video counts on the video voltage 0.3 volts to 0.0 volts about sixty times a second. If you inject DC into this, you will destroy the zero volt reference. If you come up with a circuit that will add the DC to the video, you will have to build something huge compared to a camera to make that happen. And if you have to supply AC voltage to the camera, you have the same problem made worse if the camera is color because the color sync is not exactly 60 Hz, so you would be building massive hum into your signal.

Sorry.