You don't have copy the function group if you can get all the information from the ct_bp_extern parameter.
Simply create a new function module in an appropriate group, and make sure it has the same parameters as the function you were going to copy.
You can test the values in ct_bp_extern and delete if necessary.
Then add the function in CRMC_BUT_CALL_FU and make sure it is called before the standard SAP functions.