If you want to add a widget to an already existing report you must first open the report you want to add it to. Then scroll down to the very bottom to find the icon named "Add Widget", click on it
Now you get an option to create a totally new one, or use a shared widget that already exist. In this case we will create a totally 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. The selected fields in the next picture is just an example.