There is a lot of documentation on the use of Debian/Ubuntu packaging tools, but most of these pages just list the options of each of these tools. After fighting with these for some period, I jumped over to the #ubuntu-packaging IRC channel for help. After explaining my intent of backporting some packages unchanged, I was pointed to the backportpackage tool:

 backportpackage fetches a package from one distribution release or from a
 specified .dsc path or URL and creates a no-change backport of that package
 to one or more Ubuntu releases release, optionally doing a test build of 
 the package and/or uploading the resulting backport for testing.

This tool comes with the ubuntu-dev-tools package.

I needed to backport the ocaml source package from precise (12.04) to all earlier releases up to lucid (10.04). For the backport to oneiric, here is the command:

amplidata-2.5 is the identifier to the PPA as defined in my dput.cnf file. I also configured the file ~/.devscripts with the keys DEBEMAIL, DEBFULLNAME and DEBSIGN_KEYID.