If there is no Debian package available, the general approach is to use a combination of roslocate and rosinstall. There are quite some questions on this topic here on answers.ros.org already and as makokal pointed out, there is also some information on the wiki. I will just summarize the general steps users normally do:
First, you should create an overlay. We will create it in ~/ros. An overlay is basically a local ros installation you have write permissions on. Users normally put source packages in their overlay and manage it with rosinstall. This step needs to be done only once.
rosinstall ~/ros /opt/ros/diamondback
echo "source ~/ros/setup.bash" >> ~/.bashrc
Now you can use roslocate to create rosinstall files which can then be installed. Let's assume you want to find and install the stack (or package) foo
.
roslocate info foo > foo.rosinstall
rosinstall ~/ros foo.rosinstall
The fist thing you should do now is to either re-source ~/ros/setup.bash or open a new terminal. Then just rosmake the package you are interested in.
Please note that it normally makes sense to install complete stacks, not just single packages. The reason is that stacks are units containing related packages.
When trying to build your package, rosmake might fail because of unsatisfied dependencies, so you might need to iterate the above procedure to satisfy all of them by hand.