Hello Salman,
If the external DB is not following the table structure, be it courses or learners, I am afraid just importing tables from a third party application will not do the job.
If other application supports export of user information in CSV format, then you can modify the file to match Moodle's import format and can import users this way, and course structure definitely would be entirely different from Moodle as far as I can assume. Usually people coming from other LMS's or applications are recommended to re-do the task of creating users and courses rather than trying to import and then getting into trouble later on when something breaks apart.