Of course you could add information to an already existing iso file so that it has what you need.
Just use the command "mkisofs" to turn your Linux filing system tree into an iso and burn it into a DVD.
You may need to execute mkisofs command external to the Linux so just use any Live CD. Read the man page of mkisofs command. The Grub Manual also has a good example how to burn Grub (just a single file in this case) into a CD. The step 5 of
this thread also has an example on how to make an iso for combining 7 Linux together.
In Linux you can do pretty much what you want.