Importing the target skeletal mesh into Maya or 3dsmax, etc.Open Scene Explorer, we can see “Dummy” and our mesh. Finding the missing bone, Add an Dummy with the same name and at the same level. It is best to export the source skeletal mesh as well, We can easily transform the added Dummy to the same position and rotation as the source skeletal mesh.Finally, just re-import the model,and re-designate the skeleton.