This conversion seems to be mainly related to Nero Multimedia software and used for some kind of export of Nero SoundTrax projects (.npf) to MPEG-4 multimedia files (.mp4). It seems that this is only possible with the original software and no 3rd party app will help you.
Convert Nero SoundTrax project to MPEG-4 video file format.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert npf to mp4 on Windows
A popular burning and multimedia suite from Nero.
Convert from npf 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.