Simply importing the structure of your existing db is not possible. You could however user the SDK and create a script from your existing db to create the entities, attributes and associations. See https://docs.mendix.com/apidocs-mxsdk/mxsdk/creating-the-domain-model#2-creating-entities

The data from the tables can be imported with the database replication module from the market place: https://appstore.home.mendix.com/link/app/160/Mendix/Database-replication