There is not a built in function to do this, however it can be done.
If your need is to do this interactively, the letter values can be added using the annotation tool. But I assume what you want is to have the letter values to automatically be added.
To do this, a script would need to be written to parse out the letters from the report table, and then using an Add Graphics Script, add the letters to the top of each bar.
I will point you to the Scripting Guide to get started. It is in the JMP Documentation Library, available under the Help pull down menu. It will give you the components of the language, as well as the documentation on how to work with the display trees in the report output.
The Community will be here to help you with problems you come across. Enjoy your journey.
Jim