You can definitely grind up marshmallow root in your blender or coffee grinder to get marshmallow root powder. However, unless you have a very high powered/sharp bladed grinder/blender, there tends to be a lot of fibre that will not grind left. You will likely have to sift the powder then grind and sift, grind and sift etc. until you feel like you have gotten as much powder as possible.
I have done this a few times when my usual supplier has been out of powder and while it’s possible, I found it time consuming with less usable material.