Importing Push Notification Subscribers is done as follows.

Step 1: On your left navigation menu click on Subscribers. 

Step 2: At the top middle area of your screen click the button for "Import Subscribers" 

Step 3: Select Push Notification as the platform you wish to import and add the CSV file for your push notification users that you downloaded from your previous service provider. Click Continue

Step 4:  Map Columns - Verify the fields are matched up properly. (In most cases we automatically match these with high accuracy but it's good practice to verify.

NOTE: It is good practice to select an existing Tag or to enter a new tag to segment this newly imported list.

Step 5:  Verify the # of subscribers which are being imported and click submit