You can create .deb file of any installed package on your system.
For this you will need the package dpkg-repack.
So install this package by:
apt-get install dpkg-repack
Now run by typing dpkg-repack at terminal
To generate a .deb file of rhythmbox, type this at terminal:
sudo dpkg-repack rhythmbox
This will create a .deb file in the directory where you are running the command.