The only post printing technique that I can think of was used back in film days and was the application of Impasto Gel over a print on canvas which allowed the photographer to then create brush strokes (in the Impasto) to simulate paintings. Can still be done on inkjet prints, either canvasses or printed on canvas effect paper. Hard to do convincingly and always looks a bit fake and naff (IMO) but then again, I can't draw for toffee (but will try hard for chocolate!)