It's possible that you already use a customer relationship management (CRM) system like Salesforce, MailChimp, etc.
You can integrate your existing customer data from most other platforms with AE.
We're in the process of writing out instructions for importing customer data from other services. In the meantime, check out our Import Users from Another Service article for instructions on importing users via a CSV file.
If you require more specific assistance, please send us a message.