Adding additional packages to installer ISO

Is it possible to add additional packages to the installer CD image before burning it? (Let's assume I have something that will "unzip" the ISO file into a NTFS file system, and I'll be burning straight from disk afterwards.) If I can add packages that aren't normally included on the installer CD, where would I put them, and would they be recognized either during the installation process or by Synaptic afterwards?

The issue is that I'll be installing to a computer without any internet access, and where I normally go for internet access prevents me from getting any new packages. Therefore I need to be able to carry all of the packages I wish to install with me, preferably on the same CD or DVD. So if it's possible to download the additional packages I want from packages.ubuntu.com and place them on the disc, then that's what I need to learn. If it's not possible, then why not add this functionality in the next release of Ubuntu?