The descriptions for creating a bootable USB stick usually lead
to an MBR and a FAT fileystem. Both will not suffice to boot
from CD/DVD via x86 BIOS. An EL Torito enhanced ISO 9660 filesystem
But the files in the FAT filesystem probably constitute a runnable
operating system. So you can use them as payload for an ISO which
contains bootloader equipment of ISOLINUX or GRUB2. If more than
one operating system shall be bootable, then you will have to learn
how to augment the menu configuration files of the bootloaders.
The bootloader suite GRUB2 has a program called grub-mkrescue
which produces ISO filesystem images bootable by BIOS and EFI:
grub-mkrescue -o grub.iso ...your.operating.system.payload.files...
The bootloader suite SYSLINUX has a module ISOLINUX, which
provides the equipment for booting via BIOS. The production
of the ISO filesystem image is then done by programs mkisofs,
genisoimage, or xorrisofs.
The resulting filesystem image can then be burned to CD, DVD,
or BD by the usual burn programs. Make sure you choose to burn a
pre-formatted image, rather than composing a new ISO filesystem.
Since grub-mkrescue depends on xorriso, you may use that one for
xorriso -as cdrecord -v dev=/dev/sr0 -eject grub.iso