Modifying the Standard 

You should only modify the standard SAP software if your requirements cannot be fulfilled by means of Customizing, personalization, by exits, or Business Add-Ins provided in the system. Before making modifications to the standard software, see the relevant section of the documentation of the Modification Assistant.

For more information about modifying SAP tables, see Making Changes to Tables. You can also create append structures for tables and structures with the Modification Assistant (see Modifying Tables and Structures). 

The SAP Software Change Registration (SSCR) Procedure is used for registering all modifications to SAP source code and SAP Dictionary objects.

You should also consult the sections General Development Organization and Notes on Modifying the Standard before modifying the standard system.