I would add another run for the satellite dish. Why take the chance. You can split/combine the two cables with a switch box at the HT system or TV.
When I bought my current house they had an antenna (attic) also, but it was plugged into where the cable comes into the house so it would go to all the rooms. All I did was remove the antenna and reconnect the cable TV. I then ran a separate cable from the antenna to the Audio system for the FM Tuner.