Magento 2.x.x extension attributes API add object array

Have added custom attribute to order API with following info here: https://www.atwix.com/magento-2/adding-custom-attribute-to-api-response-in-magento-2/

It adds the "customer_feedback": "this is my feedback" fine to api.

How to add an Json object array ? E.G:

"my_custom_object": {
    "Key1": "value!",
    "Key2": "value2"
}

Thanks