Assuming you have
<table id="sometableid">
<thead>
<tr>
<th>Name</th>
<th>Value</th>
</tr>
</thead>
<tbody>
<tr>
<td>A</td>
<td>1</td>
</tr>
<tr>
<td>B</td>
<td>2</td>
</tr>
</tbody>
</table>
<input type="button" onclick="addValueColumn('sometableid');" value="Add Value Column" />
and use jQuery
function addValueColumn(id)
{
$('#'+id + '> thead > tr').append('<th>Value</th>');
$('#'+id + '> tbody > tr').each(function()
{
$(this).append('<td></td>');
});
}