This is a hardware design issue. if the USB driver, the SD card and the micro-controller were not designed to function together and allow the drive to me mounted as a filesystem, then it can’t be added after the fact.
Also, at least on the Digitakt, the +Drive also can’t be mounted as a filesystem over USB either, and neither could the card on the Electribe, and the Deluge.
You could try experimenting with Air Card ( SD Card with built-in WiFi )