You can mix silver and copper metal clays to make your own alloys.
You can read about the technique at
As for getting silver clay syringe to stick to milled sterling
sheet, the shrinkage will be a problem for you. The silver clay
syringe work will shrink when fired. In order to fire on sterling, it
will have to be buried in activated carbon to keep the sterling from
oxidizing/fire scale. Alternately, you can raise a fine silver
surface by depletion gilding the sterling, but you still have the
shrinkage problem. The shrinkage will still most likely detach it
from the base piece at least in places and you will have a hard time
re-attaching it. Much easier to make the whole thing from clay.
Another good solution is to make the syringe work, fire it, then
solder it onto the sterling base.The syringe work will need to be
very well constructed, continuous without breaks.
Mary Ellin D’Agostino, PhD