Why don't you create a meta package that depends on all the packages you want to install, and just configure preseed to install it.
equivs was designed for this kind of thing, although creating a meta package from scratch with standard packaging tools is not very difficult, either.
To instruct the installer about installing packages, you can include the line
di pkgsel/include string package1 package2
in the preseed file. If you just want to install all *.deb files from a directory (not a complete Apt repository with Packages.gz , etc.), Maybe
di preseed/late_command string in-target dpkg -i /pool/extras
but I would prefer a complete repo; basically it just runs through dpkg-scanpackages .
source share