Hi rusil,
You can add this feature, edit the file: "yourSite/administrator/components/com_hikashop/views/import/tmpl/file.php" and add the code:
	<tr>
		<td class="key" >
			<?php echo JText::_('UPDATE_PRODUCT_QUANTITY'); ?>
		</td>
		<td>
			<?php echo JHTML::_('select.booleanlist', 'update_product_quantity','',JRequest::getInt('update_product_quantity','0'));?>
		</td>
	</tr>just befor the tag "</table>".
And edit the file: "yourSite/administrator/components/com_hikashop/helpers/import.php", in the function "_insertOneTypeOfProducts()" add the code:
							if(JRequest::getInt('update_product_quantity','0') && $field=='product_quantity' && $product->product_quantity != -1){
								$product->product_quantity += $already[$product->product_id]->$field;
							}just before the line:
$line[] = $this->db->Quote(@$product->$field);
By this way you can choose if you want to update the product quantity or replace it.
You can add the translation:
UPDATE_PRODUCT_QUANTITY="Update the product quantity instead of replacing it"
in HikaShop > Configuration > Languages