Where can we map data for custom segments of the Basic idoc type COPAGN01

Hi Friends,

I have created extension idoc from the basic idoc type COPAGN01 and all the required steps are done.

Question is: I am not able to populate (map) data to the custom fields of the extended idoc type,

                    Please someone reply if is there any user exit or BADI etc.

Regards

Nani