There is no direct way to import OpenUtilities models into WaterCAD or WaterGEMS. We can however import them with ModelBuilder to create pipes and nodes based on polylines and points in the DGN.
If you are using a version older than 10.04 you can simply open the ModelBuilder tool and select Bentley Design Files as the Data Source Type, then complete the Wizard steps following the instructions in the Wiki Article entitled "Using ModelBuilder to Import External Data"
If you are using WaterGEMS or WaterCAD Update 4 (version 10.04.*.*), the OpenUtilities DGN file must first be exported to .DXF format and can be imported using the ModelBuilder tool by way of the CAD File option.
In ModelBuilder you can see there are many layers (levels) for the different node types. Blocks are also turned into layers here, so some of these may be the blocks that represent certain fittings (see more here). These will need to be mapped to the appropriate node element type.
Using ModelBuilder to Import External Data