Use the "Save" button in the Structured Keywords panel. Save to a file of your choosing. Open it in a plain text editor. Study the text file and compare its layout to what you see in the Structured Keywords panel.
Items in [ ] are categories that are never part of the keyword structure but are useful for organizational purposes.
Items in { } are synonyms
Items with no surrounding meta-characters are keywords and are applied in the path.
The file is hierarchical in nature and Tab characters are used to set the depth of the path.
You can create your own text file and use the "Load" button in the Structured Keywords panel to replace the current list with your own, or you can use the "Merge" button to merge your list with the current list.
If you need to enter keywords with accented (or any non-ASCII) characters, make sure your text editor is set to use UTF-8 encoding or PM will have trouble interpreting your extended characters.
Instead of editing a text file, you can also right-click in a column to edit with choices from a contextual menu that appears. You can add items, siblings, etc. You can also remove/rename/cut/change types of items.
HTH,
-Kirk