NEPLAN projects (.nepprj) can be exported to .ndt files using the program's default export menu. This probably used to transfer data between two computers with NEPLAN installed. As far as we know, this is the only way to achieve this conversion.
Convert NEPLAN project to NEPLAN external program import or export file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert nepprj to ndt on Windows
A power engineering analysis and calculation software
Convert from nepprj 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.