A couple of ideas. The first is sweat soldering. Melt the solder
onto the back of the silver first, then place on the nickel silver
and heat until the solder flows.
2nd idea- I still wear one of the first belt buckles I ever made. It
is a desert scene composed of bent sterling wires on a nickel silver
buckle blank. Because of the difference in melting points, I was
able to fuse the silver to the nickel silver.
Dos Manos Jewelry