Third party sales with real inventory movements

We need to define in SAP process for the following scenario: the company receives order from an affiliated company (its customer) and at that point purchases the goods from external vendors. The vendor sends the goods directly to the customer, but the company wants to record real goods receipt and goods issue with accounting entries. Moreover the company serves as internal purchasing department for the group of affiliated companies and they want the employees of the end client to be able to record in SAP their order in order to have approval process in SAP.

What is the best way to configure that so data is entered only once and converts autpomatically from SO to PO or from PO to SO? I was thinking to create first SO that would automatically create PR, which would get converted to PO, but how to do the approval process?