I constantly use the pathfinder tool and I began to wonder, is there any shortcut for this? specially for the "add shapes" function.
Illustrator does not have a default shortcut for a specific Pathfinder function. Illustrator only has a shortcut to open the Pathfinder Window.
A workaround would be to a create an action for the Pathfinder functions you use the most. Then apply a F-Key to the action.
You can also repeat a Pathfinder function by pressing CMD/CTRL + 4