You can not do this the same way you can at the beginning of a project. You can however import your data with the excel importer module from the appstore: https://appstore.home.mendix.com/link/app/72/
Hi David,
As Maarten Suggested, you can import data with the excel importer module.
Adding some more documentation related to excel importer for your reference.
https://docs.mendix.com/appstore/modules/excel-importer
https://docs.mendix.com/howto/integration/importing-excel-documents
I hope this will helps you