Product Review Import/Export extension is designed to simplify some routine of Magento tasks. This extension allows you to import and export all product reviews and its relevant data like ratings, review titles, descriptions, and so on.
Import Export all Product reviews. Import/ Export other relevant information as well. Easy to add/update and delete reviews into the Magento store using CSV File. Export all the product reviews from your old website. Import the reviews to your new website easily. Imports various Reviews for many products at one time
For more detail: Magento 2 Product Review Import/Export extension