Creating a subitem by modifying the item title
Select the item.
Edit the title by pressing F2 or using the menu
Edit the text, using ' \ ' to separate the item's and the subitem's titles
Press Enter or click Ok to realize
The text on the right hand size is copied from the original item. You can change it...
... and go back to the original item to see that that still has the same text
Note that it's possible to create several sublevels at once by using multiple backslashes.