Hi Vijaya, 

Do you mean a SQL Immigration or Data Import?

In Mendix you can import Data from several ways like:

https://www.mendix.com/evaluation-guide/app-capabilities/external-data/

As for existing Database Model, you can´t do it out of box, Because mendix has a custom ORM that requires many-to-many association /relationship. 

You can check for Database connectors as well

https://docs.mendix.com/howto/integration/execute-an-sql-statement-on-an-external-database/

Best regards