No, it is not possible to register a DeviceEui (Device Extended Unique Identifier) through MQTT (Message Queuing Telemetry Transport) directly.
DeviceEui is a unique identifier assigned to a device in the context of certain Internet of Things (IoT) protocols, such as LoRaWAN (Long Range Wide Area Network). It is a fixed value associated with the physical hardware of the device and is used for secure identification and communication within the IoT network.
MQTT, on the other hand, is a lightweight messaging protocol commonly used for sending and receiving messages between devices and applications in IoT and other environments. MQTT itself does not have a mechanism for registering or managing unique device identifiers like DeviceEui.