Not sure what you’re asking really – do you want to keep the bootability and just add new files on top of that, or are you only concerned with saving some disk space, and don’t care about the previous session being lost?
As long as the disc is not finalized, you should be able to add new sessions to it. Whether the previous session contents is available or not depends more on what CD-creating tool you actually use, and how you use it.
Technically, I believe you should be able to keep the bootability of session 1 if you add another session – sessions are not watertight compartments, and session 2 can very well refer to files/directories in session 1. It would be necessary to transfer the old boot record to the new session, though. (I may be wrong – I haven’t really studied the Torito specification in detail, and there may easily be something I have missed. But as it says that the boot record is taken from the [I]last [/I]session on the CD, I see no clear technical problem in retaining bootability of session 1 while you add new data in session 2).
But as I already noted, it’s really a question if the CD-creating tool you are using is up to the job.
Nero 9, for example, does not seem to allow me to burn a bootable CD from an ISO without also finalizing the disc. It allows me to uncheck the ‘finalize CD’ check box, but it doesn’t seem to take …
Added: Or are you trying to add files to the boot image? Adding files in a new session won’t work, then. Perhaps you need to describe your problem, instead of what you think may be the solution.