It would save us much time if we can directly add the products to the category from this place o "product in category"
Currently we can just choose the category when posting the products.
You can do that with Magento.
The first step wiill be to change the filters of products grid.
By default you only will see the products that were added to the category (that means the "Yes" at the first column).
Change that filter:
You can choose "Any" for get all products or just "No" to get the products that aren't into the category yet.
Now you'll see more products.
You can check the products you want.
And now you can save you category again.
All the selected products will be added.