Can I use an extension cord with the MediCrystal Mat?

As long as it is no thinner than the cable of your controller. A heavy-duty extension cord is recommended. 

We recommend one with a built-in surge protector like this, https://www.amazon.com//dp/B01LWQ933G to protect your controller from power spikes. 

Or you can use a standard extension cord as long as the cable is no thinner than that of your mat and plug, and with a wall socket surge suppressor like this one https://www.amazon.com/dp/B00B44Y6VU