I found these handy instuctions for burning a multisession disk from the command line, works for CD and DVD.
Code:
growisofs -Z /dev/hdd=imagefile.iso
This will burn the iso file.
The to add files to it use;
Code:
growisofs -M /dev/hdd -J -R /path/to/files
You may not need the -J and -R for the Joliet and Rockridge files types.
On the install DVD I made, I can see the files and install Windows from it, but once it is installed, it can not read the extra files I added. Perhaps it has to do with the J and R options, but I got errors when I tried to use them.