The 27-year-old moves across the East Midlands from Leicester City on a season-long loan deal.
Hamer's rival for the number one jersey, Dorus de Vries, was named in the starting XI for Saturday's City Ground friendly against his former club Swansea City.
Trialist striker Nicky Maynard was also in Dougie Freedman's XI for the clash with the Swans.