How to make volumio index another USB drive

I attached a second USB storage (externally fed NTFS HDD) to my Pi. I managed to mount it alongside my first USB storage, and it is on /mnt/USB2 now. But it looks like Volumio doesn’t index it for its library.
I could not find a volumio configuration file. Anyone have any ideas what I should do?

Edit: well I suppose I worked around it by assigning the 2nd USB volume to /mnt/NAS, so that volumio will look at it. But if anyone knows the proper way to do it I’d love to hear about it.