The Billing Manager module does not come with an import utility built in to the module. SmithCart and Smith Registration Pro come with built in tools to import data. If you are using Billing Manager with SmithCart or RegPro you can use their import manager to import customers and payments.
If you are not using SmithCart or RegPro and want to import customers and invoices to Billing Manager you can use the SQL Server Import/Export wizard in SQL Server Management Studio (SSMS).
See the following link for more info on SSMS import wizard: