I wouldn't use granulated sugar here, because you might end up with a grainy, chunky frosting. Also, there is more granulated sugar in a cup than a cup of powdered sugar. If you're desperate, you can use granulated sugar here, but be prepared to add splashes of heavy cream to give the sugar something to dissolve into. If you're willing to experiment, I think you could have great results. Just be sure to use heavy cream, not milk!