The store can only have one default currency. If you're wanting to have your store in USD, but show prices in your local country, then using a currency switcher would be the most ideal setup. However; the checkout will still charge your customer in your stores' default currency (USD). It would be up to your customers' bank to process the currency conversion.
It is not currently possible to update pricing based on the going exchange rates outside of using a currency switcher app.
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