If you want to add your message into JSON you could do this:
var msg = "my message"; // this would be set somewhere else in the code
var jsonObject = {
"alert" : "{0}",
"badge" : "7",
"sound" : "sound.caf",
"msg" : msg
};
// convert the object into a string
var jsonString = JSON.stringify(jsonObject);
push.QueueNotification(new GcmNotification().ForDeviceRegistrationId(registrationId)
.WithJson(jsonString));
Edit: I changed the code so you don't have to manipulate the string. Instead you can now manipulate the jsonObject. JSON.stringify converts it into a string then.