Hi J Gill
I work for the Mill Products IBU within SAP, and was an SD consultant for several years as well. In my experience you will need to utilize the enhancements in IDOC_INPUT_ORDERS. There are several enhancements available to handle many customer specific scenarios with EDI inbound orders scenarios. It is quite common for projects to utilize these enhancements, especially in the case of mapping codes like material masters. These enhancements are all well documented in the system as to their specific usage. In addition, you can See Note 753153 - FAQ: Customer-functions in IDOC_INPUT_ORDERS
Thank you,
Paul Barney