Backport packages
You want to use the backportpackage command available in Ubuntu via:
apt-get install ubuntu-dev-tools
Read the manual documentation at http://manpages.ubuntu.com/manpages/precise/man1/backportpackage.1.html . You basically pass the source distribution with -s and the target destination with -d then the package as argument.
Example backporting python-git from Quantal to both Precise and Lucid:
mkdir workdir backportpackage -s quantal -d precise -d lucid -w workdir python-git
It will automatically download the files using dpkg-source:
dpkg-source: info: unpacking python-git_0.3.2~RC1.orig.tar.gz dpkg-source: info: unpacking python-git_0.3.2~RC1.orig-doc.tar.gz dpkg-source: info: unpacking python-git_0.3.2~RC1-1.debian.tar.gz
Make sure to also build the dependencies!
without signing
If you do not have a GPG key, you will want to disable signature. This require ubuntu-dev-tools >= 0.142 . You could install it from Quantal http://packages.ubuntu.com/quantal/all/ubuntu-dev-tools/download .
Direct link: http://ubuntu.wikimedia.org/ubuntu//pool/universe/u/ubuntu-dev-tools/ubuntu-dev-tools_0.143_all.deb (broken has of Sep 25th 2012).
Alternate link: http://mirrors.us.kernel.org/ubuntu//pool/universe/u/ubuntu-dev-tools/ubuntu-dev-tools_0.143_all.deb
0.142 adds a new --dont-sign option which disable signing entirely. So the first command becomes:
backportpackage --dont-sign -s quantal -d precise -d lucid -w workdir python-git