How can I add surge protection to an existing receptacle?

I have a recessed outlet behind my wall-mount tv. I want to surge protect the electrical line running to that receptacle.

I think you are talking about a Transient Voltage Surge Suppressor,also known as a TVSS.It is a protective device for limiting transient voltages by diverting or limiting surge current.It also prevents continued flow of follow current while remaining capable of repeating theses functions.

A TVSS is an integral component of a receptacle,providing local point of use protection of equipment when transient events occur within the home.
You can find them at an electrical supply house and will replace your existing receptacle.
Good luck