I was looking for the answer to this too. Apparently, the apt repository that hosts the prebuilt and downloadable ekosdebugger application is not available to non-stellarmate owners. However it is quite easy to download the necessary dependencies and compile it yourself.
First you will need to clone the ekosdebugger sources from github. Navigate to a suitable directory on your pi. The home will do just fine since the next commands will create a directory for the ekosdebugger sources.
cd
git clone https://github.com/knro/ekosdebugger.git
Now you should install the dependencies (I'm not sure if all of these are required - and credit to another poster for this info).
Some of them are duplicated but that doesn't really matter. It took a while to download and install all these but less than 30 minutes.
Now you can:
cd ekosdebugger
cmake .
make
And after this has completed you should have a new binary executable (an application) called ekosdebugger in the ekosdebugger directory. Make sure you are running it from the astroberry (windowed) session and not from an xterm (ssh session) from another machine as it requires an X display to run. If you try and run it from an ssh xterm (as I first did) you might see this:
astroberry@astroberry:~/ekosdebugger/ekosdebugger $ ./ekosdebugger
qt.qpa.screen: QXcbConnection: Could not connect to display
Could not connect to any X display.
When you run it from the astroberry window (from the Terminal application) you should see a window appear where you can choose which modules to run under debug sessions and then launch kstars from within the ekosdebugger window. It took a while longer than normal to startup for me.
Please ask if you get stuck or if you think I made a mistake (entirely possible).