Purchase order with multicurrency

Hello all,

I just want to ask your advice because I'm stuck with my problem. The case is (simple but hard to modify) one supplier can change the currency for the item's price. For example, for the first PO they use USD and the next PO they use GBP. I had the same case with creating sales order but that is much simpler because the basic price for item is based on company currency. I just change the rate in sales order entry. But for purchase order, I wonder the same approach can solve the problem. I've my hypothesis to solve the problem, like following:
1. When I enter the purchase pricing in the item, I add currency selector and update the supplier's currency according to the selected currency. For this solution, I wonder if there will be problem because in the default FA system supplier's currency cannot be changed in the supplier maintenance. I'm sure there are reason for this. So, for this solution, the problem to update supplier currency is my big concern. Can someone tell me why default FA not allowing currency change?
2. The purchase price is entered in company currency and I add some currency selector in PO entry. The problem for this solution is when I fill the price in company currency, I must manually convert from supplier currency to company currency.

So, what will you suggest to me? Any idea will help. Thank you.