mailing list signup without creating a WP user account?

I have a client who has been using the Constant Contact plugin along with a paid membership plugin. They have run into problems when a user that has signed up for a newsletter using the Constant Contact plugin tries to register as a paid member. When this happens, they are told that their email address is already in use, as they have had a user account created for them within WP when they signed up for the newsletter, but are not aware that this account has been created nor do they know their username or password to log in. Is it possible to use the Constant Contact plugin to register a user on a mailing list without creating a WP user account? We want to keep the newsletter entirely separate from ou WP user accounts.

https://wordpress.org/plugins/constant-contact-api/