To create a nested object, call createNestedObject(). To create a nested array, call createNestedArray().

For example:

DynamicJsonBuffer jsonBuffer;
JsonObject& root = jsonBuffer.createObject();

JsonObject& weather = root.createNestedObject("weather");
weather["temperature"] = 12;
weather["condition"] = "cloudy";

JsonArray& coords = root.createNestedArray("coords");
coords.add(48.7507371, 7);
coords.add(2.2625587, 7);

root.prettyPrintTo(Serial);

will generate:

{
  "weather": {
    "temperature": 12,
    "condition": "cloudy"
  },
  "coords": [
    48.7507371,
    2.2625587
  ]
}

The ArduinoJson Assistant can generate the skeleton for you.

See: