Adding Groups and Subtotals to Preliminary Payments Register

We have a need to add a grouping by Supplier and Site to the Preliminary Payments Register Report in payables.  The seeded rdf file that generates all the data is very large with many groups but not the groups we are looking for.  It would be awesome if we could find a way to do this in the template, but when adding groups to a report that generates XML in this way it was usually something that I would do through modifying the RDF to give the groups in XML.  I am hesitant to do this with a spawned seeded