Hello Fenil,
Coming to modifying a Standard SAP Service,
Its not a good idea to modify the Standard Service shipped by SAP.
I would suggest to copy Standard MPC and DPC classes into a new classes, then add fields in New MPC and implement custom logic in New DPC class accordingly.
Now in SPRO, Maintain Models & Services with a New Name and a New Version.
Register in GW System and assign Correct System Alias and Consume it.
This way you can keep Standard Service untouched and can keep both Standard Service and Modified Service Active.
Now in /iwfnd/maint_service, you will see 2 Services. One is the Standard Service and One is the newly created Service with a New Version.
Regards,
Ashwin