This is easy in concept, expensive in implementation. I just received a quote to do this at my house, and the hardware from the manufacturer was going to cost me about $260, not including the modulator I already had.
Essentially, 4 devices are needed.
1, A band stop filter to block the existing channel, and stop the security cam from being injected back out of the house to the rest of the world.
2, A modulator for the desired frequency, VHF ie ch3 or 4, or UHF. Some models cover a wide range of channels, some support only 1 or 2.
3, A band pass filter, keeps the modulator signal from "bleeding over" on to the adjacent channels.
4, A simple splitter/combiner. This mixes the two RF signals and feeds the existing distribution system.
The money is in the filters.
I do things a little different at my house. All of my TV’s have AUX Video inputs. I use a 4 port video distribution amp to feed the tv’s, but this does require an extra cable run to each TV.
Tin Lee provided a quick quote.
http://www.tinlee.com/CATV-Signal-Injector.php?active=1
See also:
http://www.basshome.com/product_9991_detailed.htm
Eddie