In Civil 3D you have the ability to create catchments from the Analyze tab Ground Data panel. You can either create the catchment from existing objects or from a surface. The surface option is neat and quick, but some in situations the results you get are not the best. For those time you’ll want to sketch out the catchment manually and create it from your sketch.
Regardless of the method you use you may want to pull that data into SSA.
If you have to create the catchment from objects made sure you edit the flow segment from the shortcut menu with the catchment selected. You need to at least add slopes to the segments, but there is more data you may want to include such as separating the segments into sheet flow or channel flow.
Make sure you have associated the catchment with a structure. To do so select the catchment and access the catchment properties.
Once the Reference Pipe Network Structure property is set you are ready to export, but there is currently a bug with the Edit in Storm and Sanitary Analysis tool, so instead go to the Output tab Export panel and use the Export to Storm Sewers tool.
This will generate a .stm file that you can then import directly into SSA. From the file menu expand import and select Hydraflow Storm Sewers File.
Once you import the .stm file then go to the file menu and under import select Layer Manager to import your .dwg from Civil 3D as a background.