Custom OData service in Form Template...limitation of SAP

I am creating forms using "Maintain Forms template " Fiori app .to create forms .

I want to create a standalone form with my own custom data source .

Where the form will trigger my custom OData service .

As you see in the below diagram I want my custom form to call my Custom OData service .Instead of FDP_EF_PURCHASE_ORDER_SRV .

Cant we configure custom OData service .Is this a limitation .