If sending with that 2-nd email is rare you could:
In sogo.conf set
SOGoMailCustomFromEnabled = YES;
Before sending such emails, you open "Preferences" -> "Mail" -> "IMAP Accounts".
Then edit your regular account by changing "Email" to the 2-nd one.
Now when creating new mails, or replying/forwarding SOGo will set that address in From.
When you are done using the 2-nd address you have to change that preference back.

If you use that 2-nd address regularly, you have to add it to your SOGoUserSource.
Then you will be able to choose the From field when writing an email from a drop down.
How to add a second email address to an account, depends on your source (LDAP or database).