Try utilizing the "ModifyRowset" task.
There is a Tips and Tricks Article
that I put out back in June, 2014 about how to use this Task.
Just read your Excel Spreadsheet to create your rowset variable.
Modify the Rowset variable to add the values together creating the finalize rowset you desire.
Then, write out the rowset vars to your XML.
This should get you to where you're needing.