I've been looking for documentation explaining the process to add new asset type in the Utility Network, but I haven't found any details for doing this other that simple updating the attribute domain assigned to the Asset Type field at the subtype level. Here is the list of steps that I had to do to update the attribute domain, many of them not mentioned in the documentation, and at the end the ElectricNetworkEditor map using the utility network service didn't show the new asset type:
The pictures below show some of the steps in Arcgis Pro.
Picture showing the modified domain.
Picture showing the domain assigned to the asset type field at the subtype level, also the asset group on the map doesn't reflect the new asset type (LV Handhole).
The Electric Utility Network Foundation advance modelling considerations has this information about asset types:
From the Utility Network configuration documentation: