FYI: I managed to create an add-to-cart function by inserting the add-to-cart shortcode with product-id to the end of this form action:
<form action="<?php echo $wishlist_url; ?>" method="POST">
<input type="text" name="quantity" value="<?php echo $favourite->quantity; ?>" size="3" />
<input type="hidden" name="wpsc_action" value="update" />
<input type="hidden" name="wpsc_id" value="<?php echo $favourite->id; ?>" />
<td><?php echo wpsc_add_to_cart_button($favourite->product_id); ?></td>
</form>
