how to include library from Linux ext4 partition

I have a USB harddisk that is formatted as Linux ext4 partition connected to Rpi, running Volumio (of course) :wink:
This USB HD ext4 is already auto-mounted and Volumio can access this volume from terminal screen. But how do I include this USB ext4 HD as Volumio music library? I know USB HD with NTFS or FAT32 has no issue, but how to include Ext4 partition as Volumio library? Pls anybody can help? Tks a lot.