I own 2 domains. Can I put one as a subdomain of the other?

I own 2 domains. Can I put one as a subdomain of the other?

One of my domains has been registered and in use here on Zoho for 3 years. I would like to, if possible, add my other domain as a subdomain so I can transfer the email accounts for it that were already in use, to zoho. Is this allowed? How do I do that?

Also, I currently do not have a website for the domain I wish to use as a subdomain of my other domain. If it is set up as a subdomain, can I have a separate website for it? Or only a website for the mail domain?