AFAIK, they can't do this without root reseller privileges. You may be able to set their user up in /etc/sudoers so they can do this from command line, but even then, they would need the ability to change the ownership of those accounts if their reseller user is different than what it was on their old server.
That's why he asked this feature. Frankly this is just a feature that WHM not yet provide. Suppose this feature added to WHM then:
1. WHM-Root add a Reseller option to upload cpanel-backup file with Reseller rights
2. WHM-Root detected that there is a request from WHM-Reseller to restore a cpanel-backup file
3. WHM-Root will restore this cpanel-backup, set /etc/sudoers, and change ownership to certain WHM-Reseller
For reference, DA has this feature long time ago! Not only for reseller, but DA clients able to restore their own account!
The question now, when does CPanel provide this feature like?