<html>
<head>
<script>
function newTd(){
var tab = document.getElementById("tab");
var td = tab.getElementsByTagName("td")[0].cloneNode(true);
tab.getElementsByTagName("tr")[0].appendChild(td);
}
</script>
</head>
<body>
<table id="tab" border="1">
<tr>
<td>cont</td>
</tr>
</table>
<input type="button" value="Copia td" onclick="newTd()"/>
</body>
</html>