I have a DFPlayer but I haven’t tried using the U Disk feature yet. I do know it has 2 pins for usb, so I would assume if you wired your own usb cable to those data lines, it “should” be recognized as a usb device.
And if but some miracle your computer does recognize it, you should just be able to treat it like a normal usb storage.
If you do get this to work, perhaps you can document it and post it.