Do Greeks use Roman letters for serving the same purpose as the way we use Greek letters for different values of constants?

No, because the Roman and Greek letters for constants are treated as international standards: Greek is not going to rewrite E = mc[math]^2[/math] as Ε = μτ[math]^2[/math].

The very most you’ll see is High School algebra pretending x y are the Greek letters χ ψ through strategic use of Greek cursive.