Hi,
Joomla does support users to use their email address as login name. The only requirement is that they enter their email address in the username field when they register.
If you turn on the simplified registration of HikaShop, HikaShop will remove the username field from the registration form of HikaShop and set the email address of the users as their username automatically.
And if you turn on the HikaShop plugin to redirect the Joomla registration to HikaShop, you will even have that for the Joomla registration, not only on the checkout. So in that case, as far as I can see, that plugin is just not necessary on the website.