Extend the segment for INBOUND order

Hi all,

Message type : ORDCHG

basic type      : ORDERS05

process code : ORDC

Function module : IDOC_INPUT_ORDCHG

while changing sales order(VA02) the change sequence no will send from PI side.so for this i extended the segment for ORDERS05 and added 'zseg' custom filed in VBAK level.now which user exit  will get the sequence no data and where to update the same to VBAK table and also how to add the custom segment in E1EDK01 segment through user exit.

Regards,

Gurunath