I´ve been looking for this for a while, but finally someone apparently made a D20 Pen & Paper RPG adaption for the Grimm TV-Series, complete with scenarios & campaigns: Grimm D20 RPG / Pen & Paper Rollenspiel System zur TV-Serie : Nick Burckhardt : Free Download, Borrow, and Streaming : Internet Archive
So far it only seems to exist in German, but definitely looking good. Maybe someone will translate this ?