Using USB On The Go files with the stock android music app?
If the stock music app is confined to internal storage [sdcard]
and can access any directory in /sdcard that contains media
you can try a bind mount between the [media containing directory]
in your otg-device and a specified new directory in /sdcard

mkdir /sdcard/more-tunes
su
chmod 755 /sdcard/more-tunes
mount -o bind /path-to-otg-device/tunes /sdcard/more-tunes


just remember to unmount the /sdcard/more-tunes directory if you don't need it anymore BEFORE you delete it.

Less stressful option would be a softlink in the same[ish] way.

su
busybox ln -sf /path-to-otg-device/tunes /sdcard/more-tunes


for this approach you can just delete the symlink without risking loss of media files, but i don't know if your default app will follow the symlink.