How to modify GRUB to boot from e.g.: USB or CDROM?

Question

I have an installed Ubuntu (or e.g.: Fedora).
I need to boot from a CD or a pendrive.
I don't have access to the BIOS, so I can't set it to boot from a CDROM or from an USB flash drive. But I have root access to the installed Ubuntu (or e.g. Fedora).
So I though I would somehow modify the GRUB so that after the machine "boots from the HDD", the "GRUB boots from the USB flash drive". Is this possible?

Answer 1

From http://www.supergrubdisk.org/wiki/SGD_BootScript_Usb_Boot_from_Cdrom

title Try USB HD0 boot
rootnoverify (hd0)
chainloader +1
boot

And

title Try USB HD1 boot
map (hd0) (hd1)
map (hd1) (hd0)
liveswap
rootnoverify (hd0)
chainloader +1
boot

I only know that it is possible. I do not know the specifics of how it works or how to do it so I don't know if extra steps are required to get GRUB to see the USB drive as a harddrive.