The general principle is explained at
http://sbarnhill.mvps.org/WordFAQs/Overbar.htm. You use an EQ field
with the \O switch to combine two or more characters.
The article shows making one character with an overbar by writing the
field code
{EQ \O(x,¯)}
Although the article doesn't show it, you can combine pairs of
characters like this:
{EQ \O(oo,¯¯)}
where the bar after the comma is two macrons (type Alt+0175 twice).
The curve is a little more difficult because it doesn't exist in the
ANSI character set. However, it is available in the Arial Unicode MS
font, so in Word you can replace the macron in the above code with the
character that you get by typing 2322 and then pressing Alt+X. Then
you have to select that character inside the field code, go to the
Format > Font dialog and click the Character Spacing tab, and set the
position to "Raised" by 6 points.