Creating Superscript characters?

Hey there.
I’m currently trying to create a font with a whole set of superscript letters, which we use in most abbreviations in French (like Mme, Mlle, St, Éts, Cie, Sté… where only the first, capital letter remains in normal position).
How can I assign the OpenType “superscript” value to those letters? For now they are considered “alternates” but replacing every glyph one by one instead of using GREP to “select” the letters and style them with a “superscript” character style is time-consuming and not practical at all.

Hello? Anyone?
Anything about superscript and subscript?

Hi, neither superscript nor subscript are supported / planned.