I don't know whether I understood your question and I expect many of the people who would know the answer would not understand the way you asked the question.
I think you want to start with a .iso of some Linux liveDVD system and add some extra contents to a copy of the .iso and burn that to a DVD, so that when booted the original and extra contents are all available.
Originally Posted by Disco77
Do I need to unpack the distros .iso file and drop this into a new folder with the additional content and create a new .iso file of the new root folder?
That is the usual method described for the process. I never understood why people don't use the faster method that needs less (though still a lot of) work space:
Mount the .iso (or the DVD itself) as a filesystem. Mount a small tmpfs. Mount an aufs filesystem that merges the two (so the tmpfs holds only changes and additions to the .iso). Then create a new .iso (needing that lot of work space) using the aufs as a source. Dismount the aufs, tmpfs and DVD since everything is now in the new .iso. Then burn that to DVD.
I have access to Windows and Mac so I can use which ever is better suited to the job.
Obviously Linux is better suited to the job and there are more experts reading this thread who could tell you how to use Linux. I assume you don't have a Linux system to use other than the liveDVD. Ubuntu is probably not a good choice for using a liveDVD to remaster itself, without ever installing, in a system with just one DVD drive (other Linux distributions with appropriate contortions actually manage it).
Give more thought to whether you have partition space somewhere in which to temporarily install Linux to use as a platform for remastering the DVD.