Hello V Singh,
As per my understating, you want to update "CRMD_ORDERADM_H" when posting date is initial.
I don't think so, CRM_DATAEXCHG_BADI will help you to update table, the above code will help you when data is going to replicate in ECC.
I would suggest to use "ORDER_SAVE" BADI method "check_before_save", you can write your logic to check whether record has Posting Date or not if not the you can update buffer using either FM : CRM_ORDER_MAINTAIN or CRM_ORDERADM_H_CHANGE_OW.
Note: Make sure you should not "commit" into BADI and should not call FM:CRM_ORDER_SAVE as ORDER_SAVE BADI called from FM CRM_ORDER_SAVE
Hope this will help you...
Thanks & Regards,
Nikhil