Adding 4 AET Added field to BDoc From CRM to ECC

Hi Experts,

I have a requirement with middle-ware . 

Here i need to add 4 AET (BTCUSTOMER_H) created field to Bdoc at the time of service order creation

and BDOC send to ECC .

At the time of sales order creation in Ecc , i need to replicate 4 field in VBAK table.

I never work in middle ware , so seeking help from experts .

Pls provide useful doc or steps to be followed .

In Advance Thanks.

Thanks and Regards

Alok