I would need this parameter to import users from an existing database into piwik, user password in my database are already hashed.
That would really be nice :)
Are you sure that this is working for you? Password hashes use a salt so your passwords will not validate when checked on login.
-1 since the roadmap has a plan to change the hashing algorithm, and this may complicate migration.
In that case, maybe you can use direct SQL inserts in the user table....