Hey - there's no way to mass-convert users to leads. And even via the API, this isn't possible. You can only merge a lead into a user.

You'll need to archive the users and re-import the list as leads. The default selection when importing via a csv is to create users, so you might have just missed that. Be sure to select create leads, then your import will be leads 👍

Thanks! Haven't met a product this frustrating since Salesforce 😅​