Sure, you can dome or forge a piece after casting. You’ll probably
want to use a leather or nylon hammer and various rounded punches,
anvil ends, ball peen hammer heads held in a vise, or whatever you
have that is rounded and hard, even wood. Put the back of the piece
on the rounded form, and hammer away! It may make the silver a bit
more compact and dense, which will only improve it for the polishing
phase. You could also make a one or two-part mold form with
thermoplastic, and press the piece into it in a vise or screw press.
This might be very worthwhile if it is a production design.