There are many contortionist poses that any healthy person can learn to do. With enough practice and dedication you can learn sit on your own skull in a backbend, perform autofellatio in a frontbend. I have video chatted with a very talented individual who can place his shins behind his back without assistance from his hands or arms.
 
However rotating your skull 180 degrees most likely requires a specific genetic advantage and the average healthy person could not learn no matter how much practice they put into it. Example: https://www.youtube.com/watch?v=bHjRORiMl7E

Here we see the majestic bon owl in it’s natural habitat. She is mostly creepy during the day, but at night she is nightmare fuel.