Even though GEOPAK doesn't support this directly, there is a workaround - but you need to have StormCAD available.
First export the Civil3D pipe network to LandXML format. Next, import that LandXML file into StormCAD. Finally, export from StormCAD to GEOPAK (Note: you need to run StormCAD from within MicroStation, and you need to have GEOPAK and StormCAD installed on the same machine).
Regards,
Mal