Right now, building an image with a custom package requires knowledge of live-build, which is relatively obscure and not too obvious. It needs not be this way, we can add ./configure options to do that automatically.
Proposal:
--custom-packages "bsdgames robotfindskitten gnuchess" --custom-apt-entry "deb http://ftp.example.net/debian main contrib" # (multiple entries can be specified with multiple options) --custom-apt-key "/tmp/mykey.pub" # multiple keys can be specified with multiple options