In Dokan plugin seller can add terms and condition for their store and the terms and condition will appear on the store page as a tab. Also, there will be a review tab. Now if you want a field to insert seller description in seller dashboard page then you have to add a field in the seller settings page and then you have to show the description on the store page.
To extend tab on the store page you can open- dokan/includes/function.php file. In this file, you will get this filter- apply_filters( 'dokan_store_tabs', $tabs, $store_id );
If you are a developer then you can do this otherwise you have to hire a developer to customize your plugin.