Hi Fayaz,
For the outbound or inbound IDOCs, there will be a function module associated. In that function module, SAP provides with few customer user exits. Go to that function module and search with 'Call customer-function' and you will find few customer exits. Try to find the correct user exit which has the table segment structure (EDIDD). If you are able to find this, implement the exit and try to modify the custom segment of your extension idoc type. You cannot change the standard segment names.
Thanks,
Sandy