Is it possible to divide a Hummelo Bethay by using a sharp knife and dividing the plant while its still in the ground and than digging up just a part of it?
You can, but you will still have to take that portion out of the ground to consider it divided. Here is an article that will help you to care for the plant:
https://www.gardeningknowhow.com/edible/herbs/betony/growing-betony-plants.htm