I'm sure it's technically possible to modify a robotic mower with spacers to achieve a lower cut height, but I'm not aware of the specifics of how you'd do this safely. Of course, any such modification would almost certainly invalidate the warranty.

It might be worth contacting the manufacturers directly to ask if lower cut height accessories are available, or checking specialist robotics/lawn care forums specific to a particular make of mower where others might have attempted similar modifications.