I think i have the idea now on how to do this , but now my question is would you need some permishion from the cable company? Its for a 3 floor building i already installed 4 cams and the clients would like to see just one of them on there tvs the one at the front gate for visitors. thanks...

I assume this is in a business building? Does the client own /lease the whole building? If so, you could probably use a notch filter to remove one or more channels and insert the video there with a RF Modulator and a combiner. Check with CATV distribution manufacturers for details.

The main concern for most cablecos would be that your camera signal doesn't backfeed to their system. There are products to prevent this.