If any program can convert NCB files to NDB format, it's Nick-O-Matic Design Factory from which both formats (projects) originate. If that's not possible, well, then it's not possible at all.
Convert Nick-O-Matic Design Factory project file to Nick-O-Matic Design Factory project.
We found one software available in the database for this conversion.
Microsoft Windows software - convert ncb to ndb on Windows
A graphic design program of Nickelodeon cartoon characters
Convert from ncb 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.