To add a form field, go into design view.
Place your cursor where you would like the form field to appear, then go to Insert -> Form -> Text Field
you will only need to purchase other extensions if you wish to validate the added form fields.
Do not modify any of the form field names, you can modify the labels however.
when deleting a form field, you will need to delete the validation code for that field.
for example, if you delete the comments text area:
<textarea name="Comments" class="inputText" id="Comments" onblur="if (document.getElementById('commentsServerError')) document.getElementById('commentsServerError').style.display='none'" ><?php echo(ValidatedField("contact","Comments")) ?></textarea>
you will also neeto delete the code near the top of the page to validate the comments text area:
$WAFV_Errors .= WAValidateRQ(((isset($_POST["Comments"]))?$_POST["Comments"]:"") . "",true,3);