To overline a word in MS Word, this is how to do it:
1) Navigate your cursor to where you want to insert your overlined character or word.
2) Press CTRL + F9, which will insert a set of braces
3) Type in "EQ \x \to(P)" if you want to insert an overlined character above a P, otherwise replace P with your character or word.
4) Press Shift+F9 to get your character. Or, you can just re-highlight, right click and then select toggle fields.
Be careful putting the spaces in for the command. You have to get it just right, or you'll get Error! in place of your word and it won't work.