Migrate users from a custom application DB into Drupal Is there a way (kind of best practice or tool) to import Users from a custom CMS into Drupal (with of course the all fields already set)?
I think a JSON object with my all Users' data is the only way.
Passwords are actually strings encrypted in md5 so all passwords should be regenerated by the Users (that's not a big problem).
Someone suggested JSONdata.com but the URL is not reachable.

Thanks in advance for all welcome suggestions/remarks.
Yes. The Migrate module does exactly just that.

You just need to write a migration script, which is comprised of the source (there's a lot of plugins out of the box like MySQL, CSV, JSON), the destination (there's already a User class you can extend from), and the mapping (the mapping of source fields to destination fields).

Drupal will probably have a different hashing mechanism that your CMS, so the passwords probably cannot be reused. The values can be copied as is, and just force your users to reset their passwords. There's also the Force Password Change module for that.