Our Customer Add app was developed to enter a customer 1 at a time, through the Clover Register app.
It is not a very efficient app for uploading an existing customer database.
Last we checked there were a couple apps available through the Clover App Market better suited for this task. They are:
Customer Upload, https://www.clover.com/appmarket/m/TJZ4XESAWDQP8/apps/Z6GMBJ5HCBEQA
and
XL Uploader for Customer Lists, https://www.clover.com/appmarket/apps/GM1XB5DA46GDW?clientCountry=US
We have used these successfully before though we are not affiliated with the apps or support them.