Plant SpikerBox is based on the same microcontroller as Arduino Uno, so the same coding language could be applied, which means that code could be altered and expended. I believe there are extra pins left on the board itself for those expansion projects.
How I would go around this project is that I would get a touch-sensitive or conduction module and connect it to the Arduino as an input device/sensor, through which Arduino will detect the touch. Then you need to digitalize that input and make connection with the output, which is stimulating the mimosa plant.
That part I would leave to the Plant SpikeBox, because you already have a complete circuitry for stimulating the plant on it.
Alternative is to connect the output of your Arduino to extra pins on the board and add lines in the original code to read such output and stimulate the plant. Again it comes down to little bit of programming and messing around the code.