Hi,
Well, joomla is made to use the username for the login. So HikaShop does the same.
When you use HikaShop for the registration and you configure it to be in "simplified registration" so that the customer doesn't have to enter a username, the email address he enters is copied as username and thus he can use his email address for the login. That way it's consistent with Joomla and you can use the normal Joomla login module to login with the email address.
Apparently, you're using CB but it's not doing that even though it supports login with the email address. So the simplest option is to make it so that users can login with their email address in Joomla.
You can use such plugin for that:
extensions.joomla.org/extension/authentication-email
As HikaShop uses the standard authentication system of Joomla for the login process, it should work with HikaShop too.