This is a font question/suggestion, not an Excel question/suggestion.
As such, it more properly belongs in a newsgroup that deals with fonts,
such as comp.fonts.

With a font editor, you could create a font that has whaterver
characters that you wish.

Googling similar previous previous requests indicates that x-bar is
already available in at least 2 fonts:
Font MS Reference 1
Math Ext
that you might already have. Also a multiple character approach of
underscore, followed by new line (alt+enter) then X that could be
adapted to y-hat,

Jerry

Dan Donahue wrote:

Excel is used extensively for statistical work, but does not support
specialized statistical characters. For example, the sample mean is usually
written as 'x' with a bar over it, or x-bar. The equation to approximate
regression is referred to as y-hat, the letter y with a caret (^) on top.
I believe inclusion of all of the specialized statistical symbols in the
character set would add in the presentation of statistical analysis done in
Excel.