I don't think it's a great idea. It will disrupt airflow within the duct, decreasing the efficiency of the stove. If your exhaust temps are too high at the end of the bench, you could either add more bench length or put a bell at the end of the run, before the exit chimney.
Ah, did you mean underneath the duct? After reading your name it became clear English might not be your first language. Rocks in the mass can work from what I understand, but it is incredibly important to make sure the mass is monolithic, and please, do NOT USE river rocks. The moisture in them can cause them to explode if they heat up too much. I personally wouldn't use them at all, but that's just me. By the way, what kind of insulation is on the ground under the bench? And where is the stove?