You will need dpkg-repack so the first thing you need to do is install this:
apt-get install dpkg-repack
Then simply run
For instance, to repack Rhythmbox, you would run this:
This will generate a .deb file of Rhythmbox in the directory where you run the command.
And use it like so:
sudo apt-get install fakeroot
fakeroot -u dpkg-repack <package_to_repack>
One interesting thing about using dpkg-repack is that it will install any configuration files from /etc as they are upon the current host - rather than their pristine version(s). This is either a very useful thing, or not what you would expect.
For more details of using the package please see the man-page, accessible by running "man dpkg-repack".