You cannot add new widgets to the curated reports by default.
However, you can Clone the Report as a custom report and have the widgets added.
- Open a Curated Report.
- Click on the three dots next to the name of the report.
- Select Clone Report and modify the name of the report to your preference.
- Click Copy.
Now, you can click Add widgets and Drag and Drop a chart to start building the widget.
Note : The option to clone reports are currently available from Growth plans.