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 a 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.

So the output from the Arduino connected to the touch sensor, you can connect to the RCA  input on the Plant SpikerBox. The only thing you'll have to do is to see which type of shape & input wavelength does the board recognizes. 

Then you would need to alter the code in your Arduino to produce just like that or a similar input signal, which will Plant SpikerBox read as input from another plant. 

It'll be kinda tricky to tackle the code, but you'll get the advantage of the Plant Spikebox's circuitry and stimulation code.

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 a little bit of programming and messing around with the code.