You also have this option: If you want to use an additional identity as the "main" "from" for the account, you can do the following: in the account settings, choose that address as the account, and then manually enter the IMAP and smtp servers and Ugric actual account attributes for login (username, password )
Then, if needed (if you still use that occasionally) you can set the actual email account address as an identity.
So, effectively, you are switchiing your "account" address and that of an identity.
HTH.