One approach would be to use the Substitutional Alloy Builder, see attached PNG. Highlight the amorphous SiO2 that you created, the open the Builders > Substitutional Alloy tool, and select "Ag" for the substituted element and select a concentration. Note that this substitutes random elements in the oxide with Ag atoms, instead of embedding them. However, this should not change the stochiometry of the SiO2 significantly, provided the system is large enough and Ag concentrations small enough.
