Currently Civil 3D will not export a catchment object unless is connect to a structure/junction. It should export all catchment objects regardless of if they are connected to a structure/junction.
Disclaimer: The development, release, and timing of any features or functionality described or discussed for our products in this User Feedback Forum for Autodesk Water Products and Services remains at our sole discretion. This User Feedback Forum for Autodesk Water Products and Services is not a commitment, promise, or legal obligation to deliver any functionality, is intended solely to outline and gather feedback about our general product direction, and should not be relied on in making purchasing decisions.