Since Viber has access to your phone book, then you don't have to manually add a contact to Viber. What you need to do, is create a contact on the contacts of your device, and then Viber automatically adds it to your Viber contact list.
* If the contact didn't appear on the Viber list, open Viber, press Menu Button and select Update Contacts
* If you are adding an international contact (an overseas contact), then make sure to add the country code correctly at the beginning
* To view all your contacts on the Viber, no matter they are using Viber or not, press the Menu Button and then select All Contacts (To reverse the action, press Menu Button again and select Viber Contacts)