Alien can use special diff files to help make alien packages conform to
debian policy. This is only used when you are converting to deb format. This
document briefly explains how to make them. It assumes you are familiar with
working with debian source packages.

* Use "alien -g file.rpm" to generate a "source" directory tree. 
* Make whatever changes you need to make to debian/rules, add other files,
  etc.
* Use dpkg-buildpackage to generate a standard debian .diff.gz file, and
  stick it in /var/lib/alien.
* Test "alien file.rpm" - it should use your diff. If it works properly,
  (it's best if lintian is happy with the resulting .deb too), send the
  .diff.gz file to me for inclusion in alien.

-- Joey Hess <joeyh@debian.org>