Is it safe to take magnesium, k2, vit d3, calcium, whilst on a prescribed osteoporosis treatment?

Yes to vit D and Ca: Vitamin d and calcium are generally recommended for patients with osteoporosis, even those on treatment. However, there is no reason to take magnesium. It is poorly absorbed in the gut, can lead to lower calcium and other serious side effects.

Yes, take them all. : Magnesium is important for bone health, especially as you get older. It is also important for a multitude of other things. I would continue on it. Vitamin k2 is a also important and overlooked. In japan it is considered a first line treatment. All of the supplements you have listed have very high margins of safety and can be taken together. However, at only 52, i would keep the doses moderate.