I'm not sure that you need to do anything. The currency should already be the main one in the cart of the checkout.
Anyway, you can add the code:
if(!empty($this->total->prices[0]->price_currency_id)&& $this->total->prices[0]->price_currency_id!=hikashop_getCurrency()){
$url=hikashop_completeLink('currency&task=update&hikashopcurrency='.$this->total->prices[0]->price_currency_id.'&return_url='.urlencode(hikashop_currentURL()));
header('HTTP/1.1 303 See other');
header('Location: '.$url);
exit;
}
in the file "cart" of the view "checkout" and that will change the currency.