I looked through the source code, and it looks like the physics bones are actually just Godot joints under the hood, were the joint positions are then passed to the Skeleton. I couldn't find any reference to velocity in the source code, nor anything that looked like it might be usable for applying force.
I would say that currently it is not possible. You can open a feature request on the Godot proposal repository if you want, and then it might be added in a future version of Godot.
(Side note: Welcome to the forums)
