You can add soy sauce, or any sauce you like to a rice cooker. However, if you are cooking plain rice, if the sauce is a liquid, subtract this amount from the water you are adding. For example, for 1 cup white rice with 2 tablespoons soy sauce, subtract the 2 tablespoons soy sauce from 1 cup water and add 14 tablespoons (3/4 cup plus 2 tablespoons) water instead of 16 tablespoons (1 cup).