googledocskeyboardshortcut.user.js

// ==UserScript==
// @name         Insert Ś
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       shmileee
// @match        *://docs.google.com/spreadsheets/*
// @icon         data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==
// @grant        none
// ==/UserScript==

(function() {
    document.addEventListener('keydown', function(e){
        if(e.altKey && e.shiftKey){
            switch(e.code){
                case 'KeyS':
                    insertLetter();
                    break;
            }
        }
    }, true);
})();

function insertLetter() {
	var range = window.getSelection().getRangeAt(0);
	range.deleteContents();
	var newNode = document.createTextNode("Ś");
	range.insertNode(newNode);
}