To add a new row into an existing clause or paragraph click on the three dot menu button to the side of the row closest to where you wish to add the row.
You then have the choice to duplicate the row you have selected or insert a new row above or below. You can also add a row below and indented.
This functionality exists across all content sets and works in the same way.
The short video (no audio) below demonstrates how to add and duplicate rows.