Can you tell me why you want to extract the mid surface? After you mesh the body, you can select the mid surface by coordinates of nodes. For example, if your mid surface is in XY plane, you can define "z=(0-0.01,0+0.01)", in this way you will get all node of "z=0", so you extract the mid surface.
And, It is just a thought. I have no idea about how to extract the mid surface by using ANSYS Workbench . Sorry!