Add new attribute to existing entity (MQTT Sensor)

Configuration
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Hi!

I have this Aqara Cube, for which MQTT Discovery didn’t added an entity, and I have added manualy by sending this to MQTT Explorer

homeassistant/sensor/0x00158d000289d6bf/action/config

{
  "name": "Action",
  "state_topic": "zigbee2mqtt/Aqara_Cube",
  "value_template": "{{ value_json.action }}",
  "unique_id": "0x00158d000289d6bf_action_zigbee2mqtt",
  "icon": "mdi:cube",
  "device": {
    "identifiers": [
      "zigbee2mqtt_0x00158d000289d6bf"
    ],
    "manufacturer": "Aqara",
    "model": "MFKZQ01LM",
    "model_id": "lumi.sensor_cube.aqgl01",
    "name": "Aqara_Cube",
    "via_device": "zigbee2mqtt_bridge_0x00124b00258cae83"
  }
}

Now, my problem is that I want to add a new attribute to the existing battery entity of the same sensor, but I don’t want to create a new template sensor or something different from the actual sensor. Can I add a new attribute to an existing entity in the same way as I added the entity above? But I need a name for the attribute as well… Lets say that the value of the attribute should be battery value + 10, just as an example.

Thanks!