Push to array in session

Is it possible to push an item to an array that already exists in $session?

Sure, basic example:

$session = $kirby->session();

$session->set(['products' => []]);// this is not necessary if we just want to get the session data
$products = $session->get('products');
$products[] = 'Chair';
$session->set(['products' => $products]);

Note that you can only append to an array if the session value is an array and not null, so you need to take care of this.