Adding Multiple Values to a Post Meta Key

You can insert an associative array into the post_meta_field.

Here is a quick little function to grab the data afterward (did not test):

function grab_task_name_array($post_id) {
    $new_array = array();
    $array = get_post_meta($post_id,'_task-name'); //do not put true as third parameter (this would return string and not array)
    foreach($array[0] as $key => $value)
     $new_array[$key] = $value;

    return $new_array;
}

If you wanted to store a multidimensional array, some more checks would be needed.