How to put a dash or a bar over a letter in inkscape?

I'm not sure I understand; do you want kind of stripes/dashes as a fill pattern for letters? (Pattern fill can be found under fill&stroke)

Please see the attached image.

Aha - how does it read? I just found this as mathematical glyphs: ⨱, ⨰, ⨴, ⨵ 

If it's not available in a font table you might be in the need of drawing it manually or the use of a combination of X _2

This is a possible solution: Use Mathtype to write the mathematical expression, export as eps and open in inkscape.

The LaTeXText extension is great for mathematical notation.  It converts LaTeX code to svg vectors but it hasn't been updated for Inkscape 1.0+ adnit looks like the project is dormant.  You could download Inkscape 0.92.5 and use the extension from there.  I've kept legacy software at times so I could revert to familiar functions and behaviour when I needed them. 

Another workaround, similar to your eps method, is to use an online svg generator such as https://editor.codecogs.com/.  There's a fairly simple graphical interface which I used to produce this LaTeX code

        \overline{X}^{1} \; \overline{\textup{V}} \; \overline{\texttt{X}}^{2}

and then I downloaded this svg image

You could make your own font using the letters from an already existing font with your own overline variant letters and then use that font for your math documents. Or make a batch of duplicates manually for the letters you need and save it as a math template so they're there when you create a new doc from that template.