I think you have found above thing at
http://pl-developer.blogspot.in/2013/02/how-to-add-new-table-with-model-map-to.html
Below blogger has written
as Nop Commerce uses the very first release of MVC3, database migration is not supported and you must make changes to database tables by hand. Because MVC code-first must drop and recreate your database for reflecting changes to your database.