You can somewhat "fake" a soft shadow by creating multiple clones of a hard shadow and slightly enlarge and reposition each clone, with the right transparency.
Another way, a bit more complicated, is to generate (for example through a script in XNView) e new set of artwork that is a blurred, greyscale, alpha channel version of your wheel art, then use it to place a shadow. I'm doing something like this in my Arcadeflow theme (v1.3 has hard shadow, 1.4 still in development with soft shadows from custom artworks), I can provide a simple xnview script if you like