🚀 go-pugleaf

In theory, when you create a minimum document with a footnote, and export to XML, then the export should produce the matching construction.
In practice, you'll see an error message "Content contains characters which cannot be encoded". I assume this refers to the special character that binds a footnote to its reference place.

I fear you have to import the footnotes as some special tag, and run a post processing script to convert these tags into actual footnotes.

If you have time for experimentation, you might also try these codes, maybe looking into an exported INX file gets you closer.

FNcl -- footnote element, attributes are
    FNtf -- FootnoteTextFrame / The text frame this footnote appears in
    STof -- StoryOffset / The insertion point this footnote points to

Dirk