Hello @GonzaloZupo 
How can I add the ‘import’ and ‘export’ buttons in the ‘Users’ section within the admin panel?
I’m sorry, I can’t find where those buttons came from… Could you have been running your site with a plugin that added this functionality? What version of Open edX were you using when you saw them?
There is a bulk enrollment feature available on the Instructor Dashboard part of the LMS:
Using this adds “pending enrollments” for any email addresses that don’t match up with user accounts (yet), and will email the users with a link to register a new account. Once they register, their “pending enrollment” will be converted to a real enrollment in the course. This screen is documented under 16.4.3. Enroll Learners in a Course.