If you have additional fields that you want to bring over to the invoice, the only place to do that is to copy them onto the invoice lines - there are no other fields within Xero you can create.

The process (either code or manual) that you use to create invoices in Microsoft Dynamics 365 needs to copy these values into an invoice line (or lines) that are part of the invoice. If the invoice has 0 quantity and 0 price, then these will be transferred as just descriptive lines to the invoice in Xero - Xero will display the description on the invoice but without Tax/Quantity and Price amounts.