Programs like Nokia NbuExplorer can work both with the NBU and NBF files. However it doesn't seem to be possible to convert between each another. You can just recover whatever you need from the backup, like contacts and use it in your new phone. But a sucessful conversion is unlikely.
Convert Nokia PC Suite 6.82+ backup to Nokia phone side backup file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert nbu to nbf on Windows
A free explorer for Nokia NBU files
Convert from nbu 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.