Ahhh, this needs the magic comma!
$array = ((10,11,12),(20,21,22))
$array += ,(30,31,32)
$array.count; $array | foreach {[string]$_}
When you add an array, it's flattened. So "(30,31,32) is added one element of the array at a time. By placing the comma ahead of the array you're introducing an empty element and $null isn't passed through the implied pipeline (i.e., the flattening process). That leaves the array (the 2nd element of the array) to be added as an array because the comma is an array operator.