Can existing customers register for a wholesale account?
In WordPress, each user can only have one role. This means that if you are using the WooCommerce Wholesale Pro plugin, then each user can either have the normal 'customer' role or a wholesale role, but not both.
If a logged in user with a 'customer' role tries to complete the wholesale registration form then they will be asked to sign out and create a new account.
Administrators can manually convert an existing customer to a wholesale user by changing their role. You can do this under the Users section of the WordPress admin.