DPI is just a number that indicates how big are pixels supposed to be after printing them. Since I consider this number to be mostly useless as the printing plug-in allows users to set the physical dimensions of the printed image, there is no direct way to change DPI. (Though it is very easy to change it from a script.)
Though, it will not make an image pixelly 😉, you won't see the effect at all. What you probably want is to Resample the image using the "Nearest" method and a large zoom, is that right?