This is a bad idea all the way around. There is more than the 7 channels you can see on your TV that you are inserting into the cable system. Your antenna will pick up FM transmitters, Public Service radios, amateur repeaters, cordless telephones, weather satellites, and all kinds noise that will interfere with any and all cable channels - some consistantly, and some intermittantly
But don't take my word for it, you can ask the Cable Co. for a 2nd opinion