Вы можете использовать PHP array_merge()
функцию
$categories = Category::whereNull('parent_id')->get()->toArray();
$merged = array_merge($categories, [
[
'id' => 9999,
'name' => 'How Offers',
'image' => 'http://businessdotkom.com/storage/categories/January2020/1o6nDi1kjVuwje5FiFXv.png',
'products' => ProductIndexResource::collection(Product::whereNotNull('sale_price')->get()),
]
]);
return CategoryProductsResource::collection(collect($merged));