There are several good reasons not to mix kibble with raw.
Firstly, you don't have to switch gradually. That's an old wives tale. Today you start feeding raw.. and that's that. Why dogs often have a problem adjusting is because they lack the appropriate enzymes to digest raw food. They never needed t hem in the past... so the work-around is to introduce new food items OUTSIDE of their regular meal time, so that their stomachs aren't flooded with old enzymes that are anticipating their kibble type dinner.
Secondly, to make kibble highly palatable, it is generally covered in rendered fat. This fat is highly susceptible to bacteria growth at room temperature when exposed to moisture, so that is a good reason not to mix the two.
Thridly, it's just poor culinary practice. That would be like you trying to eat a nice plate of fresh vegetables and fruit, then chase it all down with a hambuger from Macdonalds.
So don't worry about introducing raw over a period of time.
Every single dog that comes to our rescue is placed on raw immediately. We don't wean them onto raw, and they do just fine... there is only wellness.