Add option to add entities to Device-Manager Device

I use the Home Assistant Modbus integration to connect a power storage device.

The Modbus integration creates entities correctly, but no actual device in the Home Assistant device registry.

I would like to use Device Manager / Device Tools to create a virtual device and assign all related Modbus entities to it, so that the entities appear as one real usable device in Home Assistant.

Currently, I can create a virtual device and merge existing devices, but I cannot assign standalone Modbus entities to the created device.

Is this currently unsupported, or am I missing a configuration step?