Yes, with a hardware modification, Panda II can support USB Host, which is what a flash drive would use. You could stream the file via a serial bluetooth connection. Note that the bluetooth connection on a Panda cannot initiate a pairing. It is only a serial client. Your source device would have to be able to pair with the Panda. Why not use the built in SD card reader?
No, you can’t get to the flash drive with Windows Explorer while it is hooked to the Panda. You could write a Windows app to do this.