Hi,
Currently, based on User data, we are not able to defined GCS from NWD files. It was tested several times with User data. The only way to achieve this is to use empty .dgn as a "GCS seed".
Idea discussed with PlantSight Team internally in Bentley. For more details please contact me directly: mateusz.jaworski@bentley.com
@Tim Lawrence - I know you were looking at this for another user, can I get you to update this item when you have more information? Or pass the information to me and I'll update.