Hi Eventyret,
It is possible to do as you said. We have just created a new collection named “Autores” (we are doing our app in spanish) and we extended the controller with the service provide by strapi: “strapi.admin.services.user.create()” to create the user with author role, and everything is working fine.
Anyway, we would like the admin to do that process directly from strapi administration panel.
Thank you very much for your answer, as it did help to make a possible workaround that we did not think about, until we find the solution to work directly form administration panel. 
