I would like to create a PPA for package that is not in Ubuntu yet. It has a dependency on another piece of software which isn't in the official repository either, but available from another PPA. Is it possible to source that PPA into mine without manually copying over the packages for every new upload?
If you don't need the PPA users to install that package and you only need it as a build dependency, you can add that other PPA as a dependency for your PPA.
To do this, go to the PPA page on Launchpad, click "Edit PPA dependencies" and there, add the PPA from which you need the dependency: