To 'Import Product Reviews', from the store dashboard go to:
> Products > Import Reviews

From within this page there are many different options and criteria to choose from when 'Importing Reviews':-
Products in: whichever categories are selected here will be the ones product reviews will be imported to. Select no categories to import reviews to all categories/all products.
Only products that currently have fewer than X reviews: ticking the box and inputting a number, for example 5, will only import reviews to those products that have fewer than 5 reviews.
Import reviews that have images: tick this box to import reviews from products that have images.
Score: import 'All reviews', '2 or more stars', '3 or more stars', '4 or more stars' or '5 stars only'
*We recommend either '3 or more stars', or '4 or more stars' to make your reviews look legitimate*
Exclude keywords: a review will be excluded if it contains any of these keywords.
Maximum Quantity: set a maximum quantity of reviews per product from 1 to 99.
Approval: if imported reviews are set to 'Draft', you will need to manually Publish them before they will be visible on the site.
(*Don't forget to click 'Import Reviews' when you're happy with the settings*)