This works if the character with bar is already
defined in the font:
amacron..umacron. Amacron..Umacron.
Perhaps the OP wants to know how to draw a bar
over any arbitrary character.
For instance x-bar, y-bar, z-bar (color science).
1) draw a graphical bar over the character, group,
copy and paste.
2) use MathType, save the character with bar as
EPS and place as inline graphic. But this leads
in my experience to problems with the vertical
positioning.
Best regards --Gernot Hoffmann