I have not tried it, but you would theoretically need just a USB OTG cable (so that you get a female USB port that you can plug your charger into) and a file manager that supports MTP. You should be able to attach your charging cable to the OTG port, and mount the watch via MTP and just transfer Music.
I'm trying to transfer from my 645M to Galaxy S7 running 8.0.0, and no dice so far. I've tried both "mtp" and "Garmin" usb modes on the watch, and when using mtp mode the phone seems to try to mount it otg and shows an "mtp host" notification saying "Accessing files from null". There is no indication in notifications or in file mgr apps that the watch's storage was successfully mounted.
Edit: btw my phone is in fact able to mount other mass storage devices via MTP..