[Martinoptic]

Calibre may not be able to convert it but the From KFX plugin should be able to - the message is quite specific: For best results use either the From KFX user interface plugin or the KFX Input Plugin CLI for conversion. See the KFX Input Plugin documentation for more information.
It is likely because calibre doesn’t support fixed layout conversion. The From KFX plugin bypasses the calibre conversion stage.