Make three classes, for example

@pre = [A H I M N T X]; # the glyphs you want to substitute out
@post = [A.alt H.alt I.alt M.alt N.alt T.alt X.alt]; # the glyphs you want to substitute in go here
@context = [S X Z s x z]; # the glyphs that trigger your substitution go here

Then in your Contextual Alternates (calt) feature use the following code

feature calt {
sub @pre' @context by @post;
} calt;

you're quite welcome. This may not be all the code you need, but it provides the most basic functionality for what you were asking about.