Whilst not exactly possible to convert Nokia backups from .arc files to .nbu, you should be able to extract the data from both and deal with as you need. This can be done with small utilities like Nokia NbuExplorer or similar. You probably won't find a straight arc to nbu converter.
Convert Nokia backup archive to Nokia PC Suite 6.82+ backup.
We found one software available in the database for this conversion.
Microsoft Windows software - convert arc to nbu 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.