Is the Windows 7 original system disk in a USB enclosure? Is that why you want to do it from USB?
If yes then I'd do the following assuming VMware Workstation is installed under Windows 8...
Create a VM pointing to the Physical Disk in the USB enclosure and do not start the VM.
Now you should be able to use VMware vCenter Converter Standalone installed in Windows 8 to do a V2V conversion and then if necessary further configure the normal file based VM created from the one using the Physical Disk.
You could also use vmware-vdiskmanager to create a separate virtual hard disk from the virtual hard disk meta-data file that's pointing to the Physical Disk and then use VMware vCenter Converter Standalone.