If you are using JSON.NET you can call Merge on two JSON Objects.
JObject obj1 = JObject.Parse(@"{
'field1': 'value1',
'field2': 'value2'
}");
JObject obj2 = JObject.Parse(@"{
'field3': 'somevalue'
}");
obj1.Merge(obj2, new JsonMergeSettings { MergeArrayHandling = MergeArrayHandling.Concat });
string json = obj1.ToString();