That is possible already:
Just please understand that second forest pack must analyze 1st forestpack geometry, and that can become quite complex with many bushes scattered, so please use with caution(and save file before attempting), on small areas/not to high density poly objects.
How to do it:
on Your bush forest object, set Render Mode to Meshes
(1) instead of automatic, and second, but not necessary step to set Display -> Viewport "Mesh" as well (2).
Then create second Forest Object for leaves, using First Forest Object as its surface.
Hope this helps.