First you need to import your list in Shopify. There are steps to import the customers in Shopify.
Shopify admin > Customers > Import Customer
Download sample CSV file from there and Make your list as per sample file
Then add the file into the popup and save. (See Image)
After successful import, Go to Push Notification, Email & SMS > General settings > Sync Settings, There is a switch to sync your Shopify customer, Please enable it. (See Image)
After that our system will handle it. After some time your imported customers will show in our app also.