How to implement custom admin actions in non-custom-models in Django?

I'd like to build a custom admin action into a model defined by Django. Let's say I wan开发者_开发百科ted to add the action "Activate selected User(s)" on django.contrib.auth.models.User in the Django admin interface. What would be the clean and proper way to accomplish this?

I had a look at the documentation but it's just about implementing admin actions in custom models if I looked correctly.

admin.site.unregister(User)
admin.site.register(User, YourUserAdmin)