You could also convert the Schema to an XSD. You can do it through programming or using a utility like XMLSpy.
The database replication module is used for importing the data from another database, not the schema.
You might want to check out the open platform functions which are part of Mendix 6+ it might help you further with importing your legacy schema into Mendix.
What is your goal, do you need the data to remain in your existing scheme in your existing database, or would a one time migration to an new Mendix database suffice?
The first option is currently not supported. The second could be build using the Mendix SDK, but easiest is probably to manually create a new domain model in the Mendix modeler. If you run on-premises you could deploy on the existing database in a new scheme (new set of tables in the same database) and copy the data using some SQL scripts. For copying the data you can also use the database replication module.