[D-MO]

Hola, prueba con esta funcion:

<SCRIPT LANGUAGE="JavaScript">
function agrega_celda(id){
    var tbody = document.getElementById(id).getElementsByTagName("TBODY")[0];
    var row = document.createElement("TR")
    var td1 = document.createElement("TD")
    td1.appendChild(document.createElement("INPUT"))
    var td2 = document.createElement("TD")
    td2.appendChild (document.createElement("INPUT"))
    row.appendChild(td1);
    row.appendChild(td2);
    tbody.appendChild(row);
  }
</script>

saludos.