The short answer is: you can't. The removable drives. i.e. USB. MMC, SD and firewire, are skipped by Converter.
You can create virtual disk, format it and then copy your USB on virtual disk.
HTH
short overview:
1. create dd-image of the disk and store it as usb-flat.vmdk
2. create descriptorfile usb.vmdk
3. create VM that uses usb.vmdk
4. run Converter "configure machine" against the new VM