Creating Mark Alt for Non-Mark Glyphs

Hi, I am making a Devanagari font where halant + consonant regularly gets replaced by an attachable mark that attaches to the bottom of the previous glyph through base to mark attachment.

I wonder if there is any way for me to make the alt glyph of a consonant (non-mark glyph) to be a mark.

Why do you think that alternative glyph is a none mark glyph?
Give it a “_bottom” anchor and set the category to “Mark” and subcategory to “Nonespacing”. Then it is a proper Mark.