Is it possible to add a note to a ToDo in Mail.app
The only feature I was able to find is that in the list of ToDo items one can add a Notes column and enter text into there for each ToDo.

To add the Notes column to the list, view your list of ToDo's and then select the Notes item from View->Columns

I couldn't find a way to view multi-line notes for a ToDo item, but I could type option-return to insert a return character.