Additional springs to carry the load better

I have a 2000 F350 DRW. I only have the single set of springs. The truck is not new to me, but that seems to be the setup for that year. I have a buddy with a 2002 and his truck has an addition set of springs on top of his main springs. Has anybody added these? I want more load carrying capacity. When I loaded a gooseneck, for the first time today, those springs sure straightened out more than I wanted them to. I guess I could get a set of bolt on helpers. But thought I could add the springs on top and the stops to have the same ride empty, but more weight carrying capacity. Other ideas? Thanks.