mp3 to m4b conversion describes audio conversion between standard MP3 and M4B ebooks, something that should be doable with some audio converters. But it might very well be possible that these files are just renamed MP4 files. In any case, M4B files can be also created in iTunes library.
Open MP3 file Open M4B fileA dedicated audio conversion program
A music library management program - renamed to Helium (version 12) and replaced in 2016