The table names are defined in formidable/classes/models/FrmDb.php.
I wouldn't advise you do this, and I will not be able to help you get the plugin functional with this modification. The database structure is pretty engrained throughout the plugin so it is not customizable without a lot of changes throughout the code. If you'd really like to save to different database tables, you will need to maintain the same structure if you want the plugin to continue functioning.You can see the database structure in formidable/classes/models/FrmDb.php as well.
Another alternative to changing to tables would be to use Formidable hooks (frm_after_create_entry) to save the entries in an additional location. The downside is that your data would be duplicated, making it more difficult to manage if you're also updating it.