report customisation

I want to include some derived data into the report and display it via a table 

I am creating the table via the xsl but I need to enter the details via the xml output. Would anyone know how to modify the xml output to include the new data in the xml? I'm assuming I need to modify the xsd to also define a new variable. Has anybody done this before? I cannot see the help for this as it just does not load on my laptop

 

Joe