The tech is not going to install your antenna unless its on a work order, and I don't believe there is an option to have a Dish tech install a customer owned antenna.
I think you could buy an antenna through dish, and then they would install it when they install your Dish.
If you called a local dealer, they could likely accommodate you, especially if they are getting a new account from you at the same time.
You could try to pay the tech to install it once he shows up, its going to be hit or miss as some techs will not do side work because if you have issues with the antenna and you call customer service, the tech could get in trouble since he didn't put the extra work he performed on a work order.