In blender there is an addon called sapling. By turning this option on we can add a tree to the scene.
In settings we can choose leaves, but they’re simple rectangular or hexagonal shapes.
I have modeled few leaves with nice textures, grouped them and I want those simple shape leaves to be duplicates of those in group that I made.
Doubligroup is the key if I understood your question right.
Separate your leafes from the tree you made with sapling and parent your leafe-object to the leafes.
Select your leafes-Object, go to Objectpanel and activate Faces under Duplication.