In the same way in which you could use your phone number as your username on your computer. Although it's more convenient for XMPP ID and emailaddress to be the same.

Theres's nothing stopping you from having a JID username@example.com and an email username@example.com. XMPP and e-mail are different protocols that operate on different ports, so their respective clients will figure out how to send a message to the correct server.

Now, if you're asking how you can set something like this up, then you independently have to set up an XMPP server and an email server on the same domain. Or use some kind of email to XMPP bridge.