One method for stubble is to texture the actual paint when you apply it by using a stiff bristle paint brush, then paint the stubble color you need after it's fried, or to use a sort of gel medium and stipple stubble texture into that (essentally sculpting with the thicker gel medium). Or, you can add very tiny amount of epoxy to achieve the desired effect. Or, you could add actual hair, like some customizers have done.
Lots of "ors" there. But there are many ways to go about it.
Btw, if it's a resin head that have, you can etch lines and wrinkles right into the resin very easily, using exacto blades, etc. Then refine them with fine grade sandpaper, so they don't look sharp-edged. To a certain degree this can be done with vinyl, too. You just have to be careful.