In the new window click “Import CSV" button and select a csv file from your computer.
After you import CSV file all message will appear with their respective link preview (if message had an url) on the left side. On the right you will be able to delete messages you don't want to add to the group by clicking trash icon. After deciding which messages you want to add click ''Save" button. The messages will be imported to the snippet group so you can use them in the future.