Ragdoll physx: making a punch mannequin

i’m currently making a “punch mannequin” (a human body from the waist up mounted on a pole and used as target practice for punches and melee weapons in my games tutorial. I’ve but together a character and rigged it cutting off the bottom half and replacing this with a rigged pole in blender. this then stands on a cube base. i have also set up rotational constraints on the bones so it behaves more like a mannequin.
In UE4 ive imported as a physics asset and want to enable physics for the whole object BUT attach it to something at the base restricting its movement here. how do i go about this ?