The only HD channels the QAM tuner in your HDTV will get are the local HD your cableco carries.
The cable box can not be directly integrated with the QAM reception in your HDTV.
The way it would work is this: Split the cable from the wall. Send one cable to the HDTV. Send the other to the cable box. When watching local HD, switch the HDTV to its internal tuner input. When watching the cable box, switch the HDTV to the cable box input.
The other approach is to rent an HD cable box from your cableco. That way you'll get other HD channels you subscribe to (like ESPN, TNT, USA, etc.), you won't have to switch between sources when going from local HD to other channels, and you can use the other features you want; VOD, guide, etc.