Check the database replication module OR consider using WEBSERVICES
Other options will hinge largely on what you aim to do
Is the SQL a database that is normally fed through another application? or do you just need it for developement right now.
Will you run cloud or on premisses?
If the database on the Mendix app remains seperate from your existing one, which will be leading?
You can use the Database Connector module: this connector provides microflow actions to execute sql queries on any database which supports jdbc connectivity. More info can be found here: How Does Mendix Support Direct Access to an External SQL Database and Execute an SQL Statement on an External Database