SAP Inbound IDOC; Use different Key field from external system to SAP

Hi guys,

I would like to seek your help on Inbound IDOC external -> SAP

The requirement is to update 3 KNA1(SAP Customer table) fields from external system to SAP (KNA1 table). The external system is throwing a Unique Number which I found out that it is stored in KNA1-SORTL. If there is no existing customer then it has to create a new one.

My objective is to specify the KNA1-KUNNR (e.g 12345) using the given Unique no from external (e.g Cust01) which is stored in in KNA1-SORTL. Currently, using Message DEBMA06 using FM IDOC_INPUT_DEBITOR

I tried debugging but cannot find the right place to convert the Cust01 (from KNA1-SORTL) to 12345 (KNA1-KUNNR).

Need your expertise advice. Thank you so much.