It looks like some dedicated CAD / 3D modeling converters such as CrossManager, support both 3DM (3D NURBS CAD) and CGR (CATIA MultiCAX) files and can convert one to another. However, we couldn't really verify the reliability of this information.
Convert 3D NURBS CAD file to CATIA MultiCAx plug-in conversion file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert 3dm to cgr on Windows
A CAD converter with huge format support
Convert from 3dm 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.