Anet A8 Installing second extruder without changing board
That is not possible without changing to a different printer main board. The Anet A8 board has 4 integrated (A4988) stepper drivers, one for X, one for Y, one for Z and one for E (extruder 0). 


Both Z steppers are controlled by a single stepper driver (they are wired in parallel to the single Z stepper driver), there is nothing to free up nor is there to configure in Marlin without replacing the main board.