An easier way might be just to make a web page that has all the
necessary sources.list lines for all the known repositories, so
the user can just paste them into their sources.list and apt-get update.
If the page was a wiki, newly found repositories could be added easily.
Another way would be to make a debian package that updates your sources.list
with new sources.list lines for unofficial repositories. That way, you could
do: apt-get install debian-science, and the package installation script will
run apt-get update for you.