WordPress doesn't allow multiple users to share the same e-mail address, but there are certain plugins that change that behavior. This is an example workaround using the Allow Multiple Accounts plugin.
1) Install and activate this plugin: https://wordpress.org/plugins/allow-multiple-accounts/
2) Use a custom PHP function in the import to generate a fake e-mail for the imported user(s): [fake_email()]. Screenshot: https://d.pr/FREE/Yr0MOL.
3) Store the real e-mail in a Custom Field named '_email_temp'. Screenshot: https://d.pr/ERw4F9
4) Use [our API](https://github.com/soflyy/wp-all-import-action-reference) to change the users e-mail to the real e-mail after the user is saved.
Here’s all of the code you’ll need to add inside the Function Editor via All Import -> Settings: https://d.pr/ef9JAn.
That’s it. Keep in mind that this code runs for all imports, so you might consider wrapping it in an [if conditional](http://php.net/manual/en/control-structures.if.php) based on the import ID ( which you can get from $_GET['id'] for manual runs or $_GET['import_id'] for cron job imports ).