It is unlikely that you could somewhat manage to convert ARC backups to NBF. Both are backups from Nokia phones, but each of them was created for different purpose. At best you can find a utility that can extract the data from such backup but conversion is most likely not possible and wouldn't even make sense.
Convert Nokia backup archive to Nokia phone side backup file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert arc to nbf on Windows
A free explorer for Nokia NBU files
Convert from arc 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.