I'm sure it's possible to do this through the database, but I can't help you with how; nor am I sure it would be the most efficient way.
A way that would be direct and relatively efficient would be to edit /includes/templates/your_template/templates/tpl_product_info_display.php:
PHP Code:
<!--bof Product description -->
<?php if ($products_description != '') { ?>
<div id="productDescription" class="productGeneral biggerText"><?php echo stripslashes($products_description); ?>.</div>
<?php } ?>
<!--eof Product description -->
Add after stripslashes($products_description) the following: ." Your end phrase."
PHP Code:
<div id="productDescription" class="productGeneral biggerText"><?php echo stripslashes($products_description)." Your end phrase."; ?>.</div>
Note: not yet tested.