h**p://www.nu2.nu/bootcd/ - the hard way
If you use Nero - that seems able to make a bootable data CD easily using the Express interface - nero.exe /w in 5.5.8.2
(I didn't actually give it a CD, but it never asked me for a floppy even when I hit BURN)
So for data files, it's easy, but converting a non-bootable image into a bootable one - I don't know... the "hard way" migt be needed, if it's even possible.
I vaguely recall, that the session containg the boot image cannot use long filename (Joliet) extensions, so for versatility, the CD can be made multisesion - programs that make them may do this automatically - and this may be an old limitation anyway!