After a lot of searching, I've been unable to find the answer by myself, so here's my problem (with Drupal 7) :
I've been able to add new fields to my user registration form (config/people/accounts/fields) and even hide fields on the form... But I would like to do the opposite : display a field on the registration form whose only purpose would be to assign a value to another field upon validation. There would be no need for this field to be stored in the database (like a transient property in Java).
Is there a way to achieve that ? Thx