Hi,
You can create a custom field of the table "product" and of the type WYSIWYG via the menu Display>Custom fields and that will add a "specification" tab where that custom field content will display if you fill it in the product.
Note that you can also add your own tabs as explained in the "How to add a tab in the tabular view ?" section of our FAQ:
www.hikashop.com/support/documentation/106-faq.html#layo