I TV digital signal and a modem signal are two different types of signals, so to answer you in short.
No, you can't go out and buy a cable modem and just hook it up, you'll have to buy the modem call Comcast, let them know you'd like to add internet access to your current Comcast account and that you are providing your own modem. They tell you how to set it up from there.
The only constant signal to a home is the basic feed for TV, nothing else.
