From The Document Foundation Wiki
How to copy comments from one cell to another
You have attached a comment to a cell and want to reproduce it on another cell which already contains data.
- Copy the cell with the comment, using ▸ or Ctrl+C,
- Select the target cell to which you wish to attach the comment,
- Right-click and choose or go to ▸ or use Ctrl+⇧ Shift+V,
- In the Selection area, uncheck everything except Comments,
- In the Operations area, choose Add.
The comment is copied without erasing the contents of the target cell.