On NS7 users can be exported to a file using
/etc/e-smith/events/nethserver-sssd-remove-provider/S20nethserver-sssd-export-accounts
In /var/lib/nethserver/backup you’ll find users.tsv and groups.tsv.
See also nethserver-sssd — NethServer 7 documentation
There’s already a Feature request about importing users/groups to NS8: Users csv or similar import - #2 by mrmarkuz