How to Populate extended IDOC File segments?

Once you extend the IDOC, it is an enhancemnet of the standard IDOC. Write some coding in the USER-EXIT for each outbound/inbound message. Go to CMOD and search for whether the message type exists for the user exit. If the message type exists then take that particular exit in the CMOD, create a new project and write the code to map the extra fields to the segments in the extended idoc.