I have a business account on the free mail plan with my own domain. Is it possible to register an @zoho.com email address (like a personal account) as part of this account? If so, how?

I tried registering such an address separately, without attaching it to my business account, but I wasn't able to, because I already used the same secondary/contact email address for my business account, and zoho won't let me register more than one zoho account with that address.

Thanks.