Sadly there is no option to import reviews to Google Business, they used to display links to other review sites but this is no longer the case. You only option is to start collecting reviews on Google. Google's algorithm does however take into external account reviews, how much weight each side holds is a whole different question.
Since I'm based in the UK, I work with a lot of industry companies that use a website called Checkatrade, its basically a site you use to find trusty construction companies, and avoid cowboys, a link and reviews from this site helps dearly, even through some of the companies I work with have zero reviews on Google.
However, Google is really pushing people to have Google accounts and I can't see them abandoning it any time soon, there's to much money involved in advertising streams whilst looking at reviews. Because of this it would be advisable to collect reviews on Google, purely for the trust factor, ranking factors and leads that you can gain from it.
If you get a lot of leads from the sites you listed then you should carry on getting reviews on these networks, if not then your primary focus should switch to potential sites that you will get leads, and sadly Google is one of them, on
- Google Maps
- Google Plus
- Google Local Search Results
If you decide to start getting reviews for Google then you should be very careful doing so, do not be tempted to pay a company to get you X reviews, do not ask friends and family or employees, Google has various ways from browsing habits, IP address and so on to find out who is legit... bad practices is a one way ticket to the Penguin Highway.
Ask your customers to leave honest feedback but do not run any incentives to do so, again this will 'MAY' result in the review being removed and who knows if that damages your reputation with Google.
Conflict of interest: Reviews are most valuable when they are honest
and unbiased. If you own or work at a place, please don’t review your
own business or employer. Don’t offer or accept money, products, or
services to write reviews for a business or to write negative reviews
about a competitor. If you're a business owner, don't set up review
stations or kiosks at your place of business just to ask for reviews
written at your place of business.
There happens to be a few ways that you can leave companies feedback, through Google Plus or Google maps, however Google advises users to use the Google Maps route, purely because its easier on mobiles.
Add a rating & review
- Open Google Maps on your computer and make sure you're signed in.
- Search for a place.
- On the card that appears below the search box, click Write a review.
- In the window that appears, click the stars to score a place and write a review.
However, some companies opt to use G+ Business Review Page Direct Link Generator.
If you just want the elegant review stars then you don't need Google reviews to do so, only if you want the right side panel, or Maps to return results. I recorded a video tutorial on YouTube that I published on our blog awhile back, informing website administrators how to use Schema and external reviews to get shiny stars appearing in search results.
Note that the benefits of including external reviews is that say you offer a product or service, on a deeper page, you can embed reviews on that page and then cite them externally, this will return stars on deeper pages, that is proven to increase click rate. I've done this for a few customers of mine, let me know if you want me to leave a comment with some examples of it in example.