Snake case function to use as an app script extension on your Google Sheets.

snake_case.js

/**
* Save this as an app script function on yout Google Sheets and start using it.
* Usage: =SNAKE_CASE(A1)
*/
function SNAKE_CASE(str) {
  return str
    .match(/[A-Z]{2,}(?=[A-Z][a-z0-9]*|\b)|[A-Z]?[a-z0-9]*|[A-Z]|[0-9]+/g)
    .filter(Boolean)
    .map(x => x.toLowerCase())
    .join("_");
}