1) Place your html code in given file template file:

app/locale/template/email/sales/order_new.html

Example Code.:

<table cellspacing=”0″ cellpadding=”0″ border=”0″ width=”325″>
<thead>
<tr>
<th align=”left” width=”325″ bgcolor=”#EAEAEA” style=”font-size:13px; padding:5px 9px 6px 9px; line-height:1em;”>Ship Date:</th>
</tr>
</thead>
<tbody>
<tr>
<td valign=”top” style=”font-size:12px; padding:7px 9px 9px 9px; border-left:1px solid      #EAEAEA; border-bottom:1px solid #EAEAEA; border-right:1px solid #EAEAEA;”>
{{var delivery_date}}
&nbsp;
</td>
</tr>
</tbody>
</table>

2) Set your additional data in given class Mage_Sales_Model_Order(Note: As this is a very specific requirement so core file is modified you can use this trick and work from local folder as well)

Find these function sendNewOrderEmail() in line number.:1241

Prepare your data in array

Finally assign your array into mailer function

$mailer->setTemplateParams(array(
‘order’        => $this,
‘billing’      => $this->getBillingAddress(),
‘payment_html’ => $paymentBlockHtml,
‘delivery_date’ => “Your array goes here”
)