Regarding the contacts, you don’t have to import anything into Viber.

After confirming the activation code, data will be synchronized so you can make calls and send texts via Wi-Fi or 3G. To efficiently work with this app, it must be installed on both the computer and mobile devices you're trying to reach. As far as the configuration is concerned, you can set the tool to display alerts only for calls or for both messages and calls. If you plan to carry on with your normal activity on the PC but still want to keep an eye on Viber, you will be pleased to know that it supports message notifications that pop up from the system tray. With a variety of calling and messaging features, you have endless options when it comes to expressing yourself. All conversations will be real-time updated in Viber on all your devices. If you want to continue your ongoing conversations from smartphone to PC, you can use Viber for PC. Viber for Desktop is a cross-platform utility. Free download Viber for Windows 20.4.0 full version standalone offline installer for Windows PC, Viber for Windows Overview

For mac download Viber 20.3.0