You could use the HxC utilities which includes an image converter, and convert the DSK image to MFM. This will also give the bonus of the new image being able to be written to.
...Isn't writing back to the image file actually a bad thing, though? As in, it destroys its original state?
HxC is interesting, but doesn't seem amazingly fleixble when it comes to extracting data.
For example, if I have a copy protected disk, and I *know* the sector sizes on the disk are a lie, the software only allows me to load and view the image using the incorrect sizes, I can't tell it to override logic and show me / extract the full sectors ignoring the sizes.