Hi John,
I typically use the Excel Importer module to import test/sample data I’ve created using https://generatedata.com/ It’s very straightforward to setup.
https://marketplace.mendix.com/link/component/72
https://docs.mendix.com/appstore/modules/excel-importer/
This gives you full control in mapping test/sample data into an existing domain model and also supports multi-worksheet data sets where you can define associations between related data if you have more complex test/sample data sets.
Regards, Steve.