The dongle has the USB boot-loader pre-installed in the same space of the boodloader for DFU OTM, so are there some way for use it for DFU OTM?
I would suggest to read the section "Using an external debugger" carefully, along with the other sections in this blog post. Then, I would buy a header or desolder a header from a DK (for example from the debug in port). Then, you can solder the header on to P1 port of your dongle (check the HW schematics to make sure you are doing it correctly). If done properly, you should then be able to program the dongle via a dev kit via an SWD cable. That way, you will not require the usb bootloader & can rather use the mesh bootloader to do a background mesh DFU. You may need to make some changes to the boards header file to make sure the GPIOs (button & LEDs) point to the correct pin numbers to make the dongle work correctly.