"so, as my users aren't the same, i guess the response is NO"
One can make either full course backups that include users and their data in the course, or NO USER backups which still has all the content but no users ... nor their assignments, nor their forum postings, etc. Admin user can restore a backup to any category on new server that exist. Even restore a backup to an existing course.
"if the backup courses was created by the user XXX, i need to have a user named XXX on the target server to make it possible."
Superuser or admin on new server can restore courses of either type (full user backups or no user backups). That user doesn't have to be in either of those types of backups ... probably best the superuser isn't made a teacher nor a student in those backups ... conflict of roles possible (if admin account on both servers were made with same username (admin) same email address).
"why the administrateur of the target server can't appropriate the course owner-rights while he's restoring it?"
Admin user on any system is superuser ... has access rights to all.
There is one way to find out the in's and out's of backups and restores and that's to try it as the admin user.
'spirit of sharing', Ken