A feature can be added manually to the feature library from the feature panel. After expanding the side panel, find the feature you want to add to the feature library in the feature list, press the three vertical dots next to the feature, and press “Add to Feature Library” from the dropdown list.

If the library contains a full duplicate of the selected feature, there will be a “The feature already exists in the library” error message. Otherwise, the feature will be added to the library and there is a confirmation message.