Your store will always have one base currency, which you can choose in your Shopify admin. You can easily change how it displays by adjusting your currency formatting.
When you set up your default currency in the payments section, that would need to be linked to the same destination bank account. So in order to offer USD as the default currency, you would need to have a US bank account linked to your payment settings. If you were able to do that, you could then choose USD as the new currency if you wished to do so.
Hope this information is helpful for you.
If looking for default currency change then Settings-->general-->Store currency->Change formatting-->Save
If you are looking for currency converter then go for Multi Currency Converter App