You could either alter PuchPurchaseOrderDP class as Martin suggested, and later on open the design in VS and add the fields to your report design
I too faced such a scenario but i opted to create a new SSRS report as i didn't want to risk messing up the original report's configuration (a beginner to X++ development). First i replicated the data fields of that class by creating a query of my own, then designed an SSRS report using that same query. Lengthy but simple and risk free :)
