Hi,
Yes. Usually what you do is that you disable the USD currency but keep it displayed and configure BRL as the main currency. That way, when the customer browse the website with the USD currency and then arrives on the checkout, the system automatically converts the prices to BRL.
If you want to convert a price in your code using the currency converter system in place, you can do like that:
$class = hikashop_get('class.currency');
$srcCurrency = $class->get('USD');
$dstCurrency = $class->get('BRL');
$convertedPrice = $class->convertUniquePrice($price, $srcCurrency->currency_id, $dstCurrency->currency_id);