addRowToEverySheet.js

// Google Sheets Script
//
// 1. In the Tools Menu, select "Script Editor"
// 2. Paste this in replacing all text in the script editor
// 3. Click disk icon to "Save Project"
// 4. Refresh spreadsheet
// 5. Be patient and a new "Custom" menu will appear
//
// add a single row to every sheet in the current spreadsheet
function addRowToEverySheet() {
  var sheets = SpreadsheetApp.getActiveSpreadsheet().getSheets();
  for (var i = 0; i < sheets.length; i++) {
    sheets[i].appendRow([""]);
  }
}

// add a menu item to call the function that adds a single row to every sheet in the current spreadsheet
function onOpen() {
  SpreadsheetApp.getActiveSpreadsheet().addMenu("Custom", [{name: "Add row to every sheet",  functionName: "addRowToEverySheet"}]);
}