A spinner control allows you to play games such as tempest the way they were originally played. And while the X-Arcade doesn't include a spinner, it is actually quite easy for anyone to add one or more to it.
First off, you will not be able to connect a spinner to the existing X-Arcade electronics, so when looking for a spinner to purchase be sure it has its own USB connection. These are quite common.
There are basically 2 options: 1) You can get a spinner that fits right into an existing button hole. This means no drilling, but you will be sacrificing a button this way. Here is a great example of this kind of spinner. 2) You can find a spot on the X-Arcade where you want the spinner and drill a hole and mount it. If you get one that fits a button-hole you will want to buy a 1-1/8" paddle bit and drill from the face of the controller starting slowly til you get through the laminate. You can also buy other spinners that only require a 1/4" hole to be drilled.