It seems that dsn to opj conversion is typically related to attempts to convert files from OrCAD PCB Designer. This should be possible using the original program. Unfortunately, we were unable to verify whenever this conversion is actually possible or useful.
Convert OrCAD schematic capture data to OrCAD capture project.
We found one software available in the database for this conversion.
Microsoft Windows software - convert dsn to opj on Windows
A modeling suite used to design printed circuit boards
Convert from dsn file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.