You can achieve modification-free extensibility by using the Redefine Gateway Service function.
Some Fiori app development occurred prior to the Redefine Gateway Service function. These were not developed using the Service Builder tool. Therefore, not all applications comply with the rules of Service Builder. Such applications may be extended using other mechanisms, such as Business Add-Ins (BAdIs). Check the application-specific documentation in each case.
By extending an OData service you are creating a custom OData service, or OData project, that redefines an SAP-delivered one.