You will have no problems silver soldering stainless to Sterling as
long as you use EasyFlow or Tenacity flux (or their equivalents).
I’ve had limited success soldering stainless with fluxes formulated
specifically for precious metals. You can get stainless wire from a
steel stockholder and make your own posts.
Sally- Most jewelry findings suppliers will have surgical steel
posts. The trick is to get posts with a soldering pad so that no
solder contacts the client’s ears. Surgical to sterling is easy since
you don’t have to worry about melting your posts. Use silver solder
and paste flux. Don’t be surprised if the surgical posts turn black.
It’ll polish off.