Bell, Rogers, Videotron, etc ... are not going to allow you to touch their equipment. Trying to inject a modulated signal into the buildings distribution has to be done right, using the correct filters, or you're going to cause all kinds of interference and you might interrupt cable co provided services.
Since you want to do it yourself and provide this as a free service to your building, I would do this a more modern way and provide it as an internet feed of the cameras. This way everyone can use their smart phone, iPad, PC, and may new TV's have internet browsers built right in. All you would have to do is get a static IP number or use a domain with a dynamic DNS. You could also feed to a service like justin.tv Doing it this way will get around any issues you'd have with Rogers, Bell, etc and it'll be available to everyone.