Here's an idea that should work:
Make a new folder on your hard drive.
Use IsoBuster to extract the ISO to the new folder.
Add whatever files or folders you like to the new folder.
Make a new ISO of the new folder.
I'm not familiar with IsoBuster so it may not do what I suggest. If not, burn the original ISO to disc then copy the files from the disc to the new folder. Then add your new files and folders and burn them back to a disc. Now rip the disc in ISO format.
This is just a guess but it sounds logical to me.