How to Customize the BIP Invoice template data source

We are upgrading from 9.1 to 9.2 and need to convert the old Crystal Invoice to the new BIP templates in order to use the delivered email functionality and store the invoice image within PS.

However, our Invoice currently prints some addtional information (like the ship to address and supplier code coming from the customer setup data using a custom field) that is not in the default data source BI_INVOICE_DATA.

Peeplebooks section Creating New BI Publisher Invoices explains some of this. It tells us that the file layout should contain this data, but if the file layout contains the data and the data source does not (like the record BI_EXT_SHIP_TO), how do we add it to the data source in case we stick to XML. Do we need to completely build the xsd? Oracle suggested we'd create a new data source based on the current query we use in 91. But there must be a reason why they switched to XML instead of query, so I am wondering what is the best practice here.