If you want to add a widget to an already existing report you must first open the report you want to add it to.
Scroll to the bottom of the report and click Add widget:

Now you get an option to create a totally new one, or use a shared widget that already exists. In this case we will create a new widget. Enter the Title, Type and select the template Empty Widget. Then click Save:

In the next view we can select the metrics etc that we want. When we have selected what we want, we can click Save and the new widget will be added. Example: