You can use the Find existing button in the Menu bar to search for an existing topic and insert it into the open book.
- Open the book you want to insert the topic into.
- Select a topic in the left pane: the existing topic will be inserted under the selected topic.
- In the Menu bar click the button. The window is displayed.
- Click the button next to the search field to specify in which folder you want to search.
- In the search field, type the name or part of the name of the topic that you want to insert, and then click the search icon.
- Scroll through the list of results to search for the topic.
- Select a topic to display a preview in the right pane.
- Click Insert to insert the previewed topic.
- Optionally, move the topic to another location in the book using the buttons in the Menu bar.
A video for this procedure is available at: https://vimeo.com/362701817