Statistical Quantity and UOM

I am checking some functionality on GL and Payables to track statistical quantity and Unit of Measure and in both AP and GL it works fine. When I post to GL the values are carried across and for manual journals the values can be entered and also reverse when the journal is reversed. I can see no way of doing this is Purchasing however so the process I need will not work. Ideally I need to raise a PO and take the qty across to Payables when the PO is matched so that the statistical values are available in GL.