Hi,
This is a member of the Viber Development Team! :)
unfortunately, this feature is not available yet. Viber can only sync people to your Viber account if you have them only on your phone.
Meanwhile, we invite you to enter our Feature Request page and post your suggestion there, or join an existing similar suggestion.
Unfortunately it is not possible for us to integrate every suggested feature into Viber, but the more people join on a request, the faster it will become reality :)
You can find it here:
Your ticket details
If you have any question about Viber - please feel free to ask.
Thank you,
Viber Team