Hi,
Thanks for the question. At present this isn't possible using a single marker - though it's something we'll see if we can improve. You can do it with two markers, one to change the spheres to a cube, and another to change them back again to a Sphere. Please see the attached file for an example of how you can set this up.
I hope that helps, please let me know if you have any further questions.
Cheers!
Paul