HRK + PP + HNB

add_filter('woocommerce_paypal_args', 'convert_hrk_to_eur');
function get_currency() {
    $eur_rate = get_transient( 'eur_rate' );
    if ( empty( $eur_rate ) ){
        $response = wp_remote_get( 'http://hnbex.eu/api/v1/rates/daily');
        if ( is_wp_error( $response ) || 200 != wp_remote_retrieve_response_code( $response ) )
            $eur_rate = '7.5';
        try {
            $result = wp_remote_retrieve_body( $response );
            $currency_list = json_decode( $result, true);
            foreach ($currency_list as $item) {
                if ($item['currency_code'] === 'EUR') {
                    $eur_rate = $item['median_rate'];
                }
            }
            set_transient( 'eur_rate', $eur_rate, 6 * HOUR_IN_SECONDS );
        } catch ( Exception $ex ) {
            error_log( 'HNBEX API REQUEST ERROR : ' . $ex->getMessage() );
            $eur_rate = '7.5';
        }
    }
    return $eur_rate;
}

add_filter( 'woocommerce_paypal_supported_currencies', 'add_hrk_paypal_valid_currency' );
    function add_hrk_paypal_valid_currency( $currencies ) {
     array_push ( $currencies , 'HRK' );
     return $currencies;
    }
add_filter('woocommerce_paypal_args', 'convert_hrk_to_eur');

function convert_hrk_to_eur($paypal_args){
    if ( $paypal_args['currency_code'] == 'HRK'){
    $convert_rate = get_currency();
    $paypal_args['currency_code'] = 'EUR';
        $i = 1;

        while (isset($paypal_args['amount_' . $i])) {
            $paypal_args['amount_' . $i] = round( $paypal_args['amount_' . $i] / $convert_rate, 2);
            ++$i;
        }

        if ( $paypal_args['discount_amount_cart'] > 0 ) {
            $paypal_args['discount_amount_cart'] = round( $paypal_args['discount_amount_cart'] / $convert_rate, 2);
        }
        }

return $paypal_args;
}