spread.addCustomName("customName1", "=12", 0, 0);
spread.addCustomName("customName2", "Average(20,45)", 0, 0);
activeSheet.setFormula(1, 0, "customName1");
activeSheet.setFormula(1, 1, "customName2");
var names = spread.getCustomNames();
for (var i = 0; i < names.length; i++) {
    alert(names[i].getName());
}