There isn't a way to directly import the form, or even workflows to O365, but you may want to look into Sharegate for migrating Nintex objects into O365 from any version of SharePoint.

Since 10th March 2016:

- Changed the Form file type from XML to NFP

  * Exporting from the designer is now an NFP package

  * Export all forms will now include NFP files

  * XML files are still supported in import

I don't try to import the nfp form from SharePoint OnPrem to Online.

Nintex Forms for Office 365 - Release Notes

This does not work unfortunately. Only option is 3rd Party product.