You could enhance the function module responsible for the Inbound processing so that when the port is blank, you set a value to it. See the attached link for details. Notice that the function module (and the function exit) depends on the IDOC type.
http://help.sap.com/saphelp_nw73ehp1/helpdata/en/4b/4c82ac1c300a93e10000000a421937/frameset.htm