Soldering fine silver legally?

I am planning on switching to fine silver over sterling. It’s softness should really only pose a problem for prong settings (in which case I would use sterling) My question is how would I Solder this legally? Of course I could fuse the first joint but the second, third, fourth etc? Could I fuse all the joints or could I get away with legally soldering (and mainting the ability to hallmark it 999)? What If I use silver with just a touch of copper to lower the melting point slightly as solder? Would this work?