Может так
$backets=unserialize($backet);
$n=count($baskets)+count($new_element_array);
array_pad ($baskets, $n, $new_element_array);
$b=serialize($backets);
setcookie ("basket", $b, time()+7200)
или так
$backets=unserialize($backet);
array_push($backets, $new_element_array);
$b=serialize($backets);
setcookie ("basket", $b, time()+7200)
// $new_element_array - твои новые эл-ты