Yes you can, no matter if you don’t have a 3G version and here is a quick guide on how to do it.
To activate Viber on your iPad or iPod Touch, use your cellphone number when signing up. After you have received an SMS with an activation code, just enter it into your iPad or iPod Touch and it should start working immediately.
Viber was primarily developed for iPhones, so to achieve maximum experience, it is recommended that you use it with an iPhone and iOS 4 or newer. Even though Viber is known to work on iPod Touch and iPads, they are not officially supported.