One way to add an external button to act in place of button A would be to connect it to pin 14 on the Zumo 32U4 with a pull-down (1k) resistor so that when the button is pressed it pulls that I/O pin to ground through the resistor. You can look at the Zumo 32U4 schematic diagram under the "Resources" tab on its product page. However, if you want the external button to not have the same effect as pressing button A, then you will need to move it to a different pin and you might need to free up an I/O pin. The "Pin assignments" section in the Pololu Zumo 32U4 Robot User's Guide has information on what pins are used on the Zumo for which functions. If you let me know what pin you would like to connect a button to and what other features of the Zumo you plan to use in your program, I can let you know if it should work.
Please note that although button A shares the same pin as the LCD on the Zumo 32U4, you should be able to use both the button and the LCD screen at the same time.