Account email different to mailbox username?

Have just installed K9 Mail on a Galaxy S5, Android version 6.0.1

To hide the name of an email account/box on a server, I use different names. So I can’t see how to set this up in K9 Mail.

For example, server username is
K9 Mail username is obviously the same for POP3 and SMTP, and it works.

But where do I setup a default email address of the format

(I have email forwarders setup on the server of course)

Settings > [Account] > Sending mail > Manage identities

If you don’t use you can just change the initial identity. The usernames for POP3 and SMTP are stored separately (and can be edited under Settings > [Account] > Fetching mail > Incoming server > Username and Settings > [Account] > Sending mail > Outgoing server > Username).

Thanks for your reply