Webform to add row to specific sql table
Sorry, but no, you are on the wrong track.

Webform module is nice and easy, but it uses it's own storage. And whilst you can indeed use hooks to store a copy in your own table, it would take a pretty huge amount of work to do it properly.

It will be easier if you simply write your own module for that. If you don't know where to start, see Examples for Developers.