If you mean an entire paragraph, use Format>Borders & Shading to apply a Top
Border, but there is no such formatting attribute as 'overscoring' individual
characters. You can use the Line Tool to draw a line where you want it, but I
doubt that will give you the result you are looking for.
Can you be a bit more specific about what you are trying to do? Maybe use
special characters, such as Ā?
Regards |:>)