If you have an adjustable CSV you can turn it up to 60 and set the pressure switch at 50/70. Another tank is just another load for the pump to fill and will actually make the pressure lower. The flow and pressure come from the pump, not the tank. Since you have a CSV you can reduce the amount your sprinklers are putting out and will have more pressure left for the house. You will just need to run the sprinklers a little longer to get across.