The first step is to mount the ISO file to a directory.
sudo mount -t iso9660 aptoncd-20081209.iso /home/username/apt -o loop
Then add the file to the repository list, simply modify
/etc/apt/sources.list. Add the following line:
deb file:/home/username/apt/ /
From here on, we have access to the transferred packages. They will be available in all APT tools, including Synaptic.