Hi Duret,
I don't think SAP allows you to do that. In SAP you can link the MM/SD transaction and hedging instrument in the hedge plan.
What ever gain/loss you will be making in the underlying transaction , almost equal Loss/Gain you will be making in your hedging transaction (if its a effective hedge), making the rate fixed overall. But you can not change the rate of the underlying transaction, that will make hedge accounting incorrect.
Cheers !!!
Aditiya