I doubt that MS will create either such a font or an overline format,
unless they perceive a widespread need for it.
There are font editors for sale that would allow you to create your own
custom character sets.
http://www.fontlab.com/index.php
lists several. When I looked for one over 10 years ago, they were very
expensive (specialized market). Now there appears to be one that is
open source
http://fontforge.sourceforge.net/
I cannot recommend any, since my experience is extremely dated.
Depending on what you are trying to do, you might be able to get by with
far less work. Historically, most such requests in these newsgroups
have been to write statistical equations, where the bar over the letter
was shorthand for an average. For equations, the Equation editor is
easier and more flexible (Insert|Object|Microsoft Equation).
xbar is Alt-0214 in the Math Ext font from
http://www.columbia.edu/~em36/wpdos/wptoword.html
xbar is Alt-0120 in the MS Reference 1 font
Jerry