rosdep does not support adding repositories. That's a significant scope creep as well as potentially having a significant unintended impact on a running system. Right now the users are required to manually add the ROS repository, through this step they are changing the configuration of their machine. If rosdep allowed adding arbitrary PPAs you could easily end up with PPAs which provided conflicting packages and the user would no longer have control or knowledge of what software sources they had enabled for their computer.
Adding new repositories to a sources list is something which should be vetted and known about by the user/administrator. New repositories may contain more than just the packages you want, possibly including overriding existing packages on the system.
If you want to use another repository you should create your own rosdep rules file and add it to your rosdep sources. And then instruct your users to add your repository to their apt sources before using the new rosdep rules.
This also would wreak havoc on the debian building pipeline as it would have to be able to support pulling packages from the union of all the repositories, and would need to sync all required external debians from the external repositories to be able to make packages installable, because anything built into a debian package should be installable from that specific repo.
Note this is why Ubuntu's general policy is that PPAs do not contain libraries, as it's not reproducible to have PPAs chaining.
