Update Products quantity

By using your plugin. I can insert products and its subcategories:- title, SKU, Quantity and Price.
When I tried to update the Quantity of a product, it has been updated only with the updated number on the Excel sheet CSV.
What if I still have a 30 pieces in the sock, from the same product, out of 50 pieces. How can I import the new CSV file, which has another new quantity for the same product such as:- 60 pieces, allowing the new quantity amount to be added on the ones available in the stock Now, which are 30 as an example.
Shortly, how can I update the quantity using the plus calculation to add the old quantity value to the new ones in the new and updated excel sheet.