JSON stands for JavaScript Object Notation. A JSON object is really a string that has yet to be turned into the object it represents.

To add a property to an existing object in JS you could do the following.

object["property"] = value;

or

object.property = value;

If you provide some extra info like exactly what you need to do in context you might get a more tailored answer.

A JSON object is simply a javascript object, so with Javascript being a prototype based language, all you have to do is address it using the dot notation.

   mything.NewField = 'foo';