by Federica Marra.
Wow, thanks to all for your answers!
I think it is a common problem that you cannot assign system roles through the csv file.
However for my issue (more labeling than assign capabilities to the uploaded users) could work well the cohort option. I though that as for the system roles, also the cohorts were course related.
I don't need to label the users in a course level and we like that our teachers enroll their students themselves. As administrator I only create the new users with the uploading procedure, that's why I was looking to the possibility to give a system role (without capabilities) to the new users.
Hope it is clear.