Hi Matt,
Google does not take suggestions on what to post for your Google reviews. While it is true that they will pull in reviews from other sources, there is no official way to ask them to pull in reviews from multiple places. They try to pull in the reviews that are the most helpful to the user (also recognizing that a majority of visitors generally review establishments on Yelp vs. Google.)
The best thing to do from a holistic standpoint is encourage users to post reviews on whatever website they choose. More reviews=more potential to have those reviews pulled in to a Google listing.
Hope that helps!
