Copy Profit Center of one line item to other before posting in FB01

Hi

I want to copy the profit center of one line item to other line item before posting it in Fb01. Before posting the document in FB01

if at all the profit center field is not given it will give an error saying "Balancing field prctr missing for line item 004". Then v hv to maintain the default prctr for that particular GLaccount. Instead of doing this, i want the prctr field of one line item to be copied to other line items for which prctr is missing.

I searched in scn and tried using BADI's but none of them is triggered. I found v can use BTE's but I also found that by using enhancement in the FM FI_DOC_TO_ACC_TRANSFORM v can achieve this.... But i really dont know hw to do this... can anyone help me on this....