That's the end of story marker, and as far as I know you can only find the end of story location, not the marker itself, so you can find .\z which is the last character in the story (\z is thened of story location) and replace with $0\t, but that only works if there's actually text in the story. An empty story results in no change.
What are you trying to accomplish by adding the tab?
Thank you, but it doesn't work.
What, exactly, doesn't work? Adding the tab to any cell that already has text works, but as I said, cells without text are ignored. In your example above, it should work fine as long as the hyphens are in those cells. Just select the cells to which you want to apply the change before you run.
But I think this is more a workflow issue. How are you populating the cells? Do ALL cells need a tab, or are you inserting it to create that leader? Have you considered making a custom Keyboard Shortcut set and adding a key combo to insert the tab in table mode?