Not normally possible!
However, this question intrigued me and I've done a bit of research as I have not looked at the subject in a while.
Turns out there appears to be a German programmer who has built a piece of software that boots from a floppy or cd and allows one then to boot from a usb stick.
There is an excellent walk through published on howtogeek.com here.
The article has links to the necessary page for downloads.
Not tried it myself but hope it it is appropriate for your needs.