The only mp3 to dff conversion we know about, is export of standard MP3 audio files to DIFF (Direct Stream Digital Interchange) format, typically to use modern audio content in older Sony/Phillips devices.
The support for DFF format is a bit low, but you can still use Audacity or similar tools. But otherwise Internet is a bit silent about this export, so perhaps it rarely done nowadays.
Updated: September 25, 2023