The "best" way would be for the original app to look for "add-on" data in a predetermined spot.
Then when you make an addon 'myapp-contentpack-a' it should have a dependency on 'myapp'.
Finally, when you start
myapp it should just work.
FlashcardEngine installs to
/opt/com.aking1012/flashcardsEngine with a subdirectory
./data/decks. A content pack would add decks in the
A more complex example is if you wanted to add a feature. I'ld put that in the base package, but not have it "activate" unless another package is present.
Not the official answer, but it IS how I would do it