Convert a zip to a tar.gz

1. extract the zip
  # unzip -d . <file>.zip

2. repackage it as tar.gz
  # tar xzf <file>.tar.gz <dir_extracted>

or use some conversion tools, like

repackage upstream tarball

If you do repackage, make sure to provide an get-orig-source target to retrieve (?), unpack, modify, and repack that (and the top level dir should have ".orig" appended).

check diff.gz for non-debian changes