which MQTT example are you referring to, the Pusblisher or Subscriber example? Also, when you say "add custom service" do you mean adding another MQTT topic to publish/subscribe to ? Or do you mean adding a custom BLE service to the one of the examples?
I am afraid that we do not have a guide or example for adding a custom BLE service to the IOT examples.
You can use this tutorial as a reference when you create your custom service and then you will have to initialize the service ble_6lowpan_init, like we have done with the BLE ISPS Service. After you have initialized the service you will need to add the event handler to the on_ble_evt() function in ipv6_medium_ble.c file like we have done with ble_ipsp_evt_handler().
I do not think it should be that much effort to get it to work.
