Custom currency exchange vs automatic currency exchange rate for Suppliers

There are some instances where suppliers and the organisation will have contract agreements to hedge currency. 

It is great the currency exchange rate can be overridden when the purchase order is raised to honour a different exchange rate as part of the purchase.

Would it be possible to add a feature that enables a set currency exchange rate to be saved against a supplier for a period of time or permanently? This would coincide with any contractual agreements with the supplier.