It depends on what exactly it is you are trying to do.
Do you want to import users from some API or spreadsheet? Do you want to generate random users? Do you want to manually add several users at a time? Each of these are possible, but require a different solution.
Would be nice if module System would be editable. Then you could: go to your System-module’s domain model, right click User and Expose as REST resource
A name in Service, Select option ‘POST’ and go. Since System is not editable, you will have to create this rest-service manually.
Now run your app and check <project-url>/rest-doc/ where your newly created api is shown and testable.