When importing a LandXML file, catchbasins (Civil3D inlets) are imported as manholes.
If an inlet in Civil3D is set up as a "General Structure" or a "Junction Structure" (which is what most of the predefined Parts in the Civil 3D Parts Catalog are) then it will import into StormCAD as manholes, as the expected result.If you would like to import them as catch basins, they will need to be configured as an Inlet-Outlet Structure type in Civil 3D. Inlet-Outlet Structures are exported to LandXML as a "Struct" type which maps to a StormCAD Catch Basin.
Alternatively if you can identify the manholes that should be catchbasins in the imported hydraulic model, you can select them, then use the batch morph tool to convert them into catchbasin elements. See: How to Batch Morph nodes from one type to another
Setting up a parts library in Civil3D for LandXML import