Hi Suess,
If you are interested on the BADIs available for the Fiori application "My Spend", I would recommend taking a look at the following link:
App Extensibility: My Spend - SAP Fiori principal apps 1.0 for SAP ERP - SAP Library
In the case there is no standard badi for that app that applies with the changes you desire to do, I think the only solution you have is to modify the standard odata service to include the SQL statement you need.
Best Regards,
Emanuel