how to add .install file to the new feature folder?
If you stay out of the files that Features generated, you can add code to the (Features generated) module the way you normally do things.

The problem is that your module is already installed when you're adding the schema via .install. You'll need to implement hook_update_N and add the tables using db_create_table. Run update.php or drush updatedb and your schema should be created.