virtualbox.org

If the image is valid then you can't get that error. Meaning it has to be an hdd image format (not a floppy or CD format), and it has to be a format which VirtualBox supports: in practice VHD, VMDK or VDI.

this may be too simple:

1. Create a virtual hard drive of size that holds the USB image
2. Mount that virtual hard drive on the host
3. Use Disk Utility to restore the USB image onto that virtual hard drive
4. Unmount that virtual hard drive from host
5. Add that virtual hard drive to a Virtual Machine

ymmv