The man wants MORE caster. How about this idea?

Best way to do it is to just do it.
Put a doubler plate over the joint and make some half decent welds and it'll be fine. The stamped arms aren't exactly some exotic high strength tempered steel or anything. If you're really worried about the welds, get a gas torch and reheat them after welding then let it all air cool to avoid brittle areas immediately next to the weld.
With some careful measurements you should be able to figure out how much to remove to get a specific caster angle, or just go with trial and error and measure after tacking the arm together and adjust as needed.