First of, open K9 and go to your accounts settings
When in Settings, go to "Sending Mail" tab, tap on "Manage Identities".
Select "New identity" by tapping the "three dot" icon on top right. (Every disroot user by default has an email@example.com alias he can use)
And fill in the form providing the new alias address.
To change the default identity, while still in "Manage Identities" settings, just tap and hold the alias you want to set and select "Move to top / make default" option.
To send email with your new alias, just tap on the "Form" field and select alias you want to use from the dropdown menu, when composing your mail.