1.
Type whatever the character you want in cell.
Go to Insert ribbon tool -> Symbols -> Symbol.
Set Font to Arial Unicode MS.
For Character code: for bar, use 0305, for hat use 0302.
Hit insert. It will combine whatever character you typed with the diacritical marks.
2.
Use Insert -> Symbols -> Equation.
Enter your equation, using Accent to add Bar/Hat, while having character highlighted.
Note that this will be shape object and not in cell.
3. My preferred method in most cases.
Have list in UTF-8 text file. Copy and paste as needed.
If it is of interest, both the combing overline and the combining macron work as valid Excel names.
I have used x̅₁ and x̅₂ together for stats tests. Care needs to be taken to be sure to distinguish between x̅ (overline) and x̄ (macron) since, to Excel, they are completely different names.