Is there a special reason you are interested in the DNCs?
DNCs are implemented using a vector format (VPF), what we can call as well a specific SENC or internal format for displaying vector charts
VPF is not implemented in OpenCPN
More information about VPF:
Vector Product Format
What would be the benefit for OpenCPN
From the information here:
Differences Between NOAA ENCs® and DNCs®
it seems that the NOAA ENCs in S-57 format, which are supported completely by OpenCPN, are covering the same content with at least the same information.
ENCs are even more often updated than the DNCs.
From the DNC web site I understand that for the non-official users only charts produced and with copyright
from NGA or other US agencies are available.
(remember the NGA project
here some time ago...)