$ losetup /dev/loop0 /path/to/usb.img
$ VBoxManage internalcommands createrawvmdk -filename /path/to/usb.vmdk -rawdisk /dev/loop0
Please pay attention to permissions of loop device, it must be accessible by VirtualBox.
# VBoxManage extpack install Oracle_VM_VirtualBox_Extension_Pack-*.vbox-extpack