If you already have a CSV file of subscriber emails you want to import into Sumo, the rest is easy.
First, open up the Subscribers tab on the left-hand side of your Sumo Launcher.
Next, open up the dropdown menu at the top-right hand side and select Import CSV.
Click Select & Upload Your CSV and then select your CSV file.
Now you need to map columns from your CSV file to your Sumo Subscriber list.
Once everything is mapped correctly, click Start Import.
When the import is finished, you'll be prompted to refresh your Subscriber list. Do that, and you should see all of the subscribers from your CSV file in your Sumo subscriber list.