Yep, They will most likely will want to use your existing mast. They don't want to put any unnecessary holes in your roof either.
You probably have a slimline dish so the mast is a little bigger than the mast needed for the dish network dish but there are adapters they can use.
BTW: Dish networks satellites in the sky are a little further to the west from where the Directv's dish is pointing using the Western arc and a little to the east of where Direct's dish is pointing when using the eastern arc dish.