I think winiso or some other program like that can open the image and add to it without the need to extract the content, add files tothe folder and them re compress the image.
Im sure though if the image is a bootable image though i dont think this is possible, you will need to extract the iso and rebuild it with the new files added, if it is and install and you want the files installed with the bootable media without the need for your interaction is a differant kettle of fish!!
No, you can add files to the image and it still can be bootable.
But does Winiso support DVD images?