I don't use MakerBot's... But unless they sell an option for it, my guess is you can't do it. The reason is you need an extra stepper motor port on your electronics to provide power to the additional extruder. You should be able to take a look at your electronics and see if you have an extra port that looks like the ones the X,Y & Z motor are attached to. I'm going to move this down to the MakerBot section where you are more likely to get a good answer.