It doesn't seem that CDB to ARC conversion is possible. However the opposite one might and you could extract Symbian databases (*.cdb) from Nokia backups (*.arc).
Convert Symbian OS contact database file to Nokia backup archive.
We found one software available in the database for this conversion.
Microsoft Windows software - convert cdb to arc on Windows
A set of applications used to manage older Nokia mobile phones
Convert from cdb 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.