class MqttClientHelper(val mqttAndroidClient: MqttAndroidClient,
                       val mqttConnectOptions: MqttConnectOptions) {
 ...
var mqttEntries : MutableList<MqttSignalEntry> = mutableListOf()
...
}

// Желаемый вид записи в json
class MqttClientHelperJson(
        @Json(name = "uri") val serverUri: String,
        @Json(name = "connect_id") val clientId: String? = null,
        @Json(name = "options") val mqttConnectOptions: MqttConnectOptions,
        @Json(name = "topics") val mqttSignalEntryList: List<MqttClientHelper.MqttSignalEntry>
)

// Этот адаптер был проигнорирован
class MqttClientHelperJsonAdapter(val context: Context) {
    @FromJson
    fun clientFromJson(json: MqttClientHelperJson): MqttClientHelper {
        // ....
    }

    @ToJson
    fun clientToJson(clientHelper: MqttClientHelper): MqttClientHelperJson {
       //  .....
    }
}

class MqttConnectOptionsJson(
        @Json(name = "keep_alive_interval") val keepAliveInterval: Int = MqttConnectOptions.KEEP_ALIVE_INTERVAL_DEFAULT,
        @Json(name = "user_name") val userName: String? = null,
        val password: String? = null,
        @Json(name = "clean_session") val cleanSession: Boolean = false,
        @Json(name = "connection_timeout") val connectionTimeout: Int = MqttConnectOptions.CONNECTION_TIMEOUT_DEFAULT,
        @Json(name = "automatic_reconnect") val automaticReconnect: Boolean = true
)

// Этот адаптер для MqttConnectOptions отработал
class MqttConnectOptionsJsonAdapter {
    @FromJson
    fun optionsFromJson(mqttConnectOptionsJson: MqttConnectOptionsJson): MqttConnectOptions {
       //
    }

    @ToJson
    fun optionsToJson(mqttConnectOptions: MqttConnectOptions): MqttConnectOptionsJson {
       // ...
    }
}

val moshi = Moshi.Builder()
            .add(KotlinJsonAdapterFactory())
            .add(MqttAndroidClientJsonAdapter(applicationContext))
            .add(MqttConnectOptionsJsonAdapter())
            .build()

    val adapter = moshi.adapter(MqttClientHelper::class.java)

    val str = adapter.toJson(testObj)