PurchaseRequest SOAP API functionality

Summary

No updateRequisition API

Content

All,

My customer, WIX.com would like to build an external requisition creation page (from various reasons), import the requisition to Fusion SS Procurement, initiate an approval process and have the ability to withdraw the requisition from Approval, update and re-submit.
Currently the SOAP WS supports 

1. Create Requisition as ExternallyManaged='Yes' in APPROVED status (no way to initiate the approval process), with ability to CANCEL the requisition line
2. Create Requisition as ExternallyManaged='No' in INCOMPLETE status and initiate the approval cycle, but without ability to update the Requisition. 

Does anyone know of a way to update a requisition through SOAP or any other way?