How to make bootable CDROM or USB from floppy?

I have a bootable floppy that boots just fine on old PCs. I want to boot it however on my 2015 Macbook Pro, which does not have the BIOS (whereas my 2013 MBP did). So I cannot on the 2015 model even boot a Linux CD. I have an ISO header that will let me create a readable CDROM, but not a bootable CDROM. (Command is: cat isoheader.img floppy.img > cdrom.iso) The Apple bless command gives an error when I add the --legacy switch. What to do? PS: I cannot use bootcamp as I do not have enough free drive space.