Whilst the Nokia NbuExplorer can be used to extract the contents of both NBF and NBU backup archives, it is unlikely it can convert one to another. This is most likely not possible at all.
Convert Nokia phone side backup file to Nokia PC Suite 6.82+ backup.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert nbf to nbu on Windows
A free explorer for Nokia NBU files
Convert from nbf 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.