Can I add a field to a WaterGEMS layer without corrupting the feature class or geodatabase?
Yes, you can add your own custom fields to a layer in your feature class but, it needs to be done through the WaterGEMS interface. What you'll need to do is click on the WaterGEMS toolbar and find the Tools menu. Off the Tools menu you should look for User Data Extensions and select it. This will open the user data extension manager where you can create a field for any one of your WaterGEMS 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 FAQs