The short answer is: you can do whatever you wan to do! 😉 The question is: should you? And the answer is: no! Here is why:
From time to time customers ask me how it’s possible to aggregate data from third party sites like Google My Business or Facebook Pages.
I normally say that they can do that by using their own field types. Because SNIP has no built-in support to aggregate data from any sources, you need to code that yourself. This is often not an easy undertaking. The APIs are usually very complex and it takes a lot of code to achieve this.
However, you don’t need all of this because Googles reference page for reviews tells you that you should not aggregate third party ratings anyway.
Here is the cite as for May 2020:
If the review snippet is for a local business or an organization, you must follow these additional guidelines:
Sites must collect ratings information directly from users and not from other sites.
Of course this is different for products. Because there are third party tools that aggregate ratings from products (like stamped.io), you can indeed use them. But you should not for your business and/or local organization