Is it possible to add a field to a WaterGEMS, SewerGEMS, or HAMMER layer without corrupting the feature class or geodatabase?
To add a custom field to a layer in a feature class, it would need to be done through the OpenFlows interface. In ArcGIS Pro, click on the WaterGEMS-Tool tab. In ArcMap, click the WaterGEMS/SewerGEMS toolbar and find the Tools menu. From here, select User Data Extensions. This will open the User Data Extension manager where you can create a field for any one of your WaterGEMS or SewerGEMS elements. Details about how to use this feature can be found in the help documentation by searching the keywords "User Data Extension" or can be found here for "Creating Formula Based User Data Extensions".
WaterGEMS for ArcGIS Pro and WaterGEMS for ArcMap FAQs
SewerGEMS for ArcGIS Pro and SewerGEMS for ArcMap FAQs