$client = new SoapClient('http://mywebsite.com/api/v2_soap/?wsdl');
$session = $client->login('username', 'password');

$quoteId = $client->shoppingCartCreate($session);

$customerData = array(
    "firstname" => "Klevis",
    "lastname" => "Miho",
    "email" => "kmiho@myemail.com",
    "customer_id" => 2522,
    "mode" => "customer",
    "website_id" => "1"
);


$resultCustomerSet = $client->shoppingCartCustomerSet($session, $quoteId, $customerData);

$result = $client->shoppingCartProductAdd($session, $quoteId, array(array(
    'product_id' => '336',
    'sku' => '110757',
    'qty' => '1',
    'options' => null,
    'bundle_option' => null,
    'bundle_option_qty' => null,
    'links' => null
)));

$result1 = $client->shoppingCartInfo($session, $quoteId);

var_dump($result); //returns true