If the amount of data allows you, I would export your existing database tables to Excel and use the Excel Importer to insert the data in you Mendix database.
If you've used Postgres so far the db migration will be simple to any other Postgres environment or to Mendix Cloud:
1)Open PGAdmin
2)Navigate to your database
3)Right click it and select backup
4)Go through the backup process and restore the resulting file to your new environment.
To what sort of environment are you trying to move it?