Get customer list from Wp users

First up, great, great plugin.
Howbeit,
Accounting:
When I want to create an invoice or receive payment, the accounting requires creation of customers, but I already have such a list(that are my customers) under the default WP user creation system as a custom role before I happened on your plugin.

How can one, rather than create customers under accounting, get the list from WP’s users or a particular user role under the default WP user system?
Thanks.