Exchange rate in child table

In Purchase Order, if I want to create with a different currency, Say, my default currency is BDT and I want to create Purchase Order with USD. In system, I have to set exchange rate in parent fields, and items child tables rate and amount will be calculated based on the exchange rate I given in parent field. Now I want to add exchange rate in child table. So that I can change items exchange rate item wise. As exchange rate applied in taxes and totals named file. How can I implement this?