pk4 provides an easy way to fetch the sources of packages currently installed on the system. In order to let pk4 provide these packages as Git repositories you can enable gbp's unpack hook:

mkdir -p ~/.config/pk4/hooks-enabled/unpack/
ln -s /usr/share/pk4/hooks-available/unpack/gbp ~/.config/pk4/hooks-enabled/unpack/
      

This will make sure packages are imported into a git repository after download.