Hi again!,
continuing from the previous post, the base idea is to create two types of ‘arms’, one for he normal output axis and the second for the rear shaft. To easily connect it, we are going to use a hub (https://www.sparkfun.com/products/12543)
The full leg kit will look like this, more or less (it’s still a beta), but you can figure :
But looking the concept of @Alexandre and the ideas of @Matthieu from the post (http://poppy.discourse.group/t/special-foot-with-a-double-rotation/71) some modifications will be made 
As you can see our idea is to obtain a 6DOF leg. Any advise about the stability of the structure?