There is a converter, though it is not very high quality yet: gdal with the ogdi driver is indeed capable to convert
DNC maps to a multitude of other formats. I can confirm that it is able to recognize DNC.
Unfortunately it often chokes on some data elements, but getting it to production quality is just a matter of reporting (or fixing) the bugs.
I could figure out the thing through following that link. It clearly states that DNC is a VRF format. Now I could search for software recognizing VRF, which led me to the ogdi driver, and that led me to gdal. Figuring out the correct parameters took some guessing however. You should refer to your chart in the following way:
gltp:/vrf/[full pathname of directory containing the chart, including a file named "cat"]
Also you should put the chart in a filesystem which is case-insensitive, like msdosfs.