if custom tags are saved to the musicbee library, that file is separate to the settings file
c:\users\<your id>\appData\Roaming\MusicBee\MusicBeeSettings.ini
and the library is in the root folder of your music library folder: MusicBeeLibrary.mbl
If they are saved to the music file, then you just need to set up the custom tag mapping in the Tags(1) preferences