[roger64]

Thank you for taking the time to reply me.

You are right, the plugin already exists with Sigil... Its name is FNLinker. It creates new ePub3 footnotes under two conditions:
- an easy one, the presence of the semantic tag (see above)
- a tricky one (depends on the book): there must be an equal number of class anchors and class definitions.

If there is a discrepancy between these two classes, it may require a careful (long) analysis to correct the mistake(s) before being able to process the links in some few seconds.