somebody might have a better idea, but till then, you might have to do it the extended way. put the layer you want to shadow in a seperate group, duplicate it three times, give the topmost one a image mask using the shape directly below it, change the color of the topmost shape to black or whatever color your shadow should be, create a rough mask on the inside of the topmost shape to represent the border of the shadow, change that mask to subtract, then feather it..... as i said, somebody might have a better idea...