Make sure you have the extension pack installed for USB support etc.
http://www.oracle.com/technetwork/server-storage/virtualbox/downloads/index.html

When performing an unattended installation of Windows with an Autounattend.xml on a pen drive, you can accomplish this on by activating your pen drive for use within your virtual machine.

When starting your virtual computer, there are some icons in the bottom right corner. Right click the USB icon to activate your pen drive: