for Preeseed it looks like you can add other packages..
Preseeding other packages
Run scripts, and load config files
Their docs could be a bit more informative....
the Example preseed config script
also shos how to add additional packages.
# Individual additional packages to install
#d-i pkgsel/include string openssh-server build-essential
Preseed Looks like a stripped down version of FAI. If you can't figure out how to get preseed to handle your custom package selection take a look at FAI for Debian, it should be able to do what you want...
FAI is an automated installation tool to install or deploy Debian GNU/Linux and other distributions on a bunch of different hosts or a Cluster. It's more flexible than other tools like kickstart for Red Hat, autoyast and alice for SuSE or Jumpstart for SUN Solaris. FAI can also be used for configuration management of a running system.
You can take one or more virgin PCs, turn on the power and after a few minutes Linux is installed, configured and running on all your machines, without any interaction necessary