Sure,,, Just connect the Kobo to a pc and load up your epubs.. they will still be there after the owner registeres,,,, assuming they don't initiate a device reset, (not normally part of the initial setup.)
If you want the Kobo to actually be readable before registration, you can follow the instructions to bypass registration.. I personally keep a kobo sql file on hand that I can just copy to any kobo and get it up and running.. but.... it's honestly easier just to connect to wifi and register with a completely fake e-mail addres....
this_is_silly@kobo.com would would just fine, for example. It's never verified, so use your immagination.