Linking drop-ship purchase orders to sales orders

Can anyone help with this, as I'm completely stuck?

I'm using web services to create a drop-ship purchase order from an existing sales order. I can create the PO just fine, but am unable to link the two records together. I have tried:-

            var po = new PurchaseOrder
            {               
                createdFrom =  new RecordRef
                {
                    internalId = "1234562", //internal id of sales order
                    type = RecordType.salesOrder,
                    typeSpecified = true
                }, 
            //snip - rest of details here...          
}

This creates the purchase order but not the link.

I've also tried retrospectively setting the purchase order details on the sales order item, but get the dreaded "field is available either when a