The IDMAP dump file can be restored using the following command:

net idmap restore <full_path_and_tdb_filename> < dumpfile.txt

Where the Samba run-time tdb files are stored in the /var/lib/samba directory the following command can be used to restore the data to the tdb file:

net idmap restore /var/lib/samba/winbindd_idmap.tdb < idmap_dump.txt