Integration between over the air transmissions and your Dish Network set top box is entirely dependant on the set top box.
The box may have a button you have to hit on the remote, you may have to scan for all the channels, you may have to do something else. Check you owners manual on the Dish box you own as it should explain it. More often though, manufacturers integrate local OTA channels seamlessly with satelite channels and you simply tune to an OTA channel with the remote and it shows up on screen.
Terk (
www.terk.com) makes a ton of antennas that will fit exactly the need you are looking for. The antenna you are talking about that straddles your outdoor dish is manufactured by Terk and information about it is on their website. Most important is matching the antenna with your local broadcasting towers - not getting a pretty antenna that fits over your dish. OTA broadcasts typically require a fairly large room mounted antenna (you can try it in your attic) that is motorized to point in the correct direction.