Creation of Inbound Idoc for the Outbound Idoc in same client

Hi,

My requirement:

Whenver an PO is created an Outbound IDOC is generated in system ABC with client 100.I want to pass this IDOC data to create an Inbound Idoc for Sales Order(SO) in the same client.

It would be helpful if anyone can answer on how to Transfer the Outbound Idoc after it crosses the port.

I found there is a field for function module in WE21,can this be used to get the requirement done.

Please suggest.

Regards,

Amar