Hi Joan -
I understand your concern about mixing metals since the back of the earring will come into contact with the wearer's ear. Personally I wouldn't add non-sterling components to sterling posts. Here are a couple of options:
Flatten a sterling silver bead cap like #24-928,#43-547-07 or #43-547-05 to create a 10-13mm round filigree to do exactly what you're asking.
Use a
bezel-cup earring post instead of a flat pad post. I would fill the bezel with epoxy and set the button (with shank still attached) into the epoxy.
Use titanium 10mm flat-pad posts (#31-000). The posts themselves are nickel free, but since the pad is stainless steel, I would recommend coating the back of the pad with clear nail polish as extra insurance against metal allergies.
Good luck! I hope they turn out well.