1. mount the iso file to copy the data
Code:
mkdir /mnt/dsl
mount -o loop -t iso9660 dsl.iso /mnt/dsl
2. copy the files to a new read/write location
Code:
cp -r /mnt/dsl /tmp
3. Add your files to /tmp/dsl
4. Remake the iso with your new files:
Code:
mkisofs -o /tmp/newdsl.iso -b /tmp/dsl/isolinux/isolinux.bin -no-emul-boot /tmp/dsl
5. burn /tmp/newdsl.iso
EDIT:
I forgot you must test the iso on a virtual machine before you burn it.