Is it possible to fetch the total quantity of an item in a report?

for example

I have a report with all the requested items, I grouped this report by description. So if 3 people ordered 2 apples each I will have 6 apples as total for this item. Then I have a Purchase Order form with a subform to add the products. There is a lookup field in the subform. I will like that when I select apples in the lookup field then number 6 is fetched to the quantity field of this row in the Purchase Order's subform.

How can I do this?