The app cannot transfer actual music files between devices due to copyright material transfer legal issues. The only current way to do this is to import the same files to the other device. Assuming you must have had the files at some point when you originally imported them. I can add a feature request to see if we could make the import folder the app uses available to the files app. When the app was originally written 13 years ago, there was no such thing as a files app and no one has ever made this request until now.