How to dynamically add key value pairs in Kotlin object?

I have Spring Boot REST api which returns JSON object like this -

{
  "id": "1"
  "timestamp": "2020-08-03T08:49:02.679+00:00"
  "message": "hello"
}

This is directly derived from @Entity like this -

fun getMessage(id: Long) = messageRepository.findById(id)

Here, Spring boot automatically converts the response into JSON. However, I want to add another key-value pair in the getMessage() function. Similar to this - How can I add a key/value pair to a JavaScript object? but in Java/ Kotlin.

How do I do that?

Maybe you could create another Model for that and create an extension to convert into the new model.

data class Message(
    val id: Int,
    val timestamp: String, 
    val message: String   
)

data class MessageWithMoreInfo(
    val id: Int,
    val timestamp: String, 
    val message: String,
    val info: String   
)

fun Message.toMessageWithMoreInfo(val info: String) = MessageWithMoreInfo(
    id,
    timestamp,
    message,
    info
)