It is unlikely you could somehow convert .nbu backups to .nfc backups. However using specialized Nokia utilities like Noki or NbuExplorer you may extract useful content (like contacts) and import that to another device. But typically backups cannot be converted between each other.
Convert Nokia PC Suite 6.82+ backup to Nokia PC Suite phone backup file.
We found 2 software records in the database related to .nbu to .nfc file format conversion.
Microsoft Windows software - convert nbu to nfc on Windows
A small usefull program for exploring and extracting of files created with Nokia PC suite
Convert from nbu file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned nbu to nfc. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A free explorer for Nokia NBU files