Volumio doesn’t use the upstream Raspbian initramfs, so it isn’t going to be as simple as flashing Volumio to a USB drive and it Just Works™. Since it uses a Readonly squashfs + a overlayfs, things are a bit more… involved when it comes to the intiramfs. There are some discussions over on Volumio/Build about how to support USB boot – do check them out 
EDIT: Links to the discussions…