Back to Back Requisition using SOAP or API Web Service

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Hello Friends

Is that possible to create a Purchase requisition for Back to Back Sales Order using webservice?

From a quote we are already creating a Sales Order and a Purchase requisition but those are not connected in system. But actually requisition is for that Sales Order. We want to create a requisition for a Back to Back Sales Order but I want to tell which supplier to use rather than system decides based on sourcing rule.