If you have the Pro version you can can use motion blur and animation. Just animate the 'fast moving object' from a point before where you want it to appear in the end-still, to somewhere beyond that. And still render just the 'current frame'. It's in the help files (pdf) that come with the prog. That is what I used for the example above. You can also do it in post. Render the still, remove all objects but the one you want to appear moving, render again, same size, same resolution, without any light or fancy rendering stuff (just tick the box to render an alpha map, choose Photoshop as the file format). If you transport the alpha map from the second render to the first, you can make a selection of it, and make a copy of the opbject on a new layer. You can then choose to use motion blur on that layer (or multiples) and experiment with different layer blending modes and opacities. You can also add a mask to the motion blurred layer and paint away the blurring in places where it does not seem appropriate.