All Boot Camp does is create a NTFS partition for you and provide you with the necessary drivers (Boot Camp 4.x doesn't support XP and Vista anymore). In order to install Windows 7 using a USB Flash Drive to the Boot Camp partition, for instance, you'd need to play in the BIOS settings like you would on a normal PC (boot from USB HDD first, etc.) but that is, of course, not possible on a Mac, simply because Macs don't have a BIOS.
There is, however, something similar to BIOS in Macs, which is supposedly even better, called OpenFirmware, but unfortunately that is only in old Macs that did not have Intel processors. So in short, I do not think it is possible.
Visit this article for very helpful information regarding the matter:
Where’s my Mac BIOS? (How to get into OpenFirmware Easily) | blandname