Hello VIj,
In case it is not yet solved for you:
You have to create assign you custom FM created under te BTE events for the event DE-EIOUT. This will make sure that when the idoc gets created in ECC for CRM ( for ex: Customer change) then before it moves to CRM these FM will be called and the data will be adjusted as per the csutom logic before you send it to CRM.
The code is: FIBF . You can define you mapping FM both in ECC and CRM under this tcode.
-Abhishek