Didn't think you could do it, because the .40 barrel you would think would have a larger outside diameter than the opening in the 9mm slide, but apparently that's not true. Seems to be you can shave down an internal part on the slide and the cartridges should be able to feed. Also, seems to be popular thinking that you need an expensive conversion barrel to change from .40 to 9mm but that doesn't appear to be true as Storm Lake's rep has stated that the conversion and replacement barrels are the same. Check out this link:
http://mp-pistol.com/boards/index.php?showtopic=26633
http://mp-pistol.com/boards/index.php?showtopic=26633