I tried to find answers googling how to add files onto the vacant space on a live distro cd /DVD, but sadly did not find answers.

Finally, I experimented it today successfully, I would like to share this knowledge about how not to waste the vacant space on a bootable live linuxmint DVD & internet bandwidth as well. I mean you can use the space on DVD to copy the package files (.debs) which you like to install immediately after installing your favourite Linuxmint distro even without going online.

Here's how I was able to do it:

************ INITIAL SESSION *****************

>>>/dev/dvd differs for different PCs. find out what it is for yours before proceeding further! For mine it is /dev/sr0

growisofs -speed=4 -Z /dev/dvd=yourchoicebootablelinux.iso

************ SUBSEQUENT SESSIONS *********


### dummy write
growisofs -dry-run -speed=4 -M /dev/sr0 -r -R -J -joliet-long -iso-level 4 -d -D -ldots -allow-leading-dots -allow-multidot -allow-lowercase -l -L -allow-multidot -V "Actual LABEL of linux ISO" -v /path/to/additional/files

See if you get any errors. If all is fine. go ahead with the real DVD burn by issuing the following command.

### REAL write operation
growisofs -speed=4 -M /dev/sr0 -r -R -J -joliet-long -iso-level 4 -d -D -ldots -allow-leading-dots -allow-multidot -allow-lowercase -l -L -allow-multidot -V "Actual LABEL of linux ISO" -v /path/to/additional/files