Hum... If your usb key is composed like the windows dvd, you can transform it in iso file. First, you need to download Power ISO and, if you don't have, a windows 8 iso from Microsoft. Open the windows iso in Power ISO, go to Actions, Boot, Extract boot information. After, open img burn and select create image file from folder. In Advanced, Bootable disc, select Make Image Bootable, click on browse and select the boot image you had extracted from windows iso. After, just select your files to put them in your iso. normaly, it will boot if you test under vm, if it works, just burn it