It is possible to go back into dropshadow settings and change the dropshadow applied.
Another option if you want a separate dropshadow object is create an individual frame and apply the shadow to that. If the object is an irregular shape, you can use detect edge clipping path , then press control on the clipping path and select convert clipping path to frame. (or you can just draw the path) Apply shadow to this frame and make sure the frame has a white fil. (it helps to duplicate the original image frame before converting the clipping path to frame)
John
What I'm doing is putting a simple blue bar across thetop of a brochure. I want to give the effect that the blue bar is completely justified on the page, so we can see the shadow's origin. However, I need the blue bar to be long enough to allow for bleeds.
I've done this in AI, and it looks great.