How do I add custom data to a transformer?

You can add a custom field using the add() method and returning an array:

\Aero\Admin\Transformers\ProductTransformer::add(function ($data) {
   /* @var \Aero\Catalog\Models\Product */
   $product = $data['product'];

   return [
       'foo' => 'bar',
   ];
});

You can add a cast using the addCast() method:

\Aero\Admin\Transformers\ProductTransformer::addCast('acme', 'integer');