Adding an outdoor OTA antenna is the easy part, if your dish is installed high enough on your roof to see over the peak to get line-of-sight to the broadcast towers. You'll need a "J" pole that will attach to the dish pole to attach the antenna above the dish. The incoming OTA cable can be diplexed (you'll need two or more of these) onto the incoming satellite cable, so you need not run a second cable for OTA. Running cable is not fun.
It gets a little more complicated at the in-house end, depending on how many of your 6 TV sets you want to receive the OTA signal. At your in-house splitter sight or TV sets, you'll need to use other diplexers in reverse to separate back to OTA and cable signals before they are attached to the DVR or TV sets. With 6 tv sets, you're going to need an amplified splitter to send OTA signal all over your house.
Go to antennapoint.com and determine where your broadcast towers are located, UHF or VHF or both and how far away the are, so you can get an idea of where on your roof you want the dish mounted and what type of antenna you'll need.
Whether or not Dish will deliver your locals in HD, the OTA antenna will, and almost completely uncompressed, unlike Dish. You're going to find out that you'll be watching your local HD programming over-the-air and not on Dish.
As to your satellite setup, I'm an antenna guy, so I'll leave that part to the salellite experts.
