I’ve found a workaround for this.
I have created an alias toto of user1@tst.lan .
In sogo/settings i have defined an imap account toto@tst.lan and then i can select toto@tst.lan from the menu when i send a mail.
I agree that it is not possible to do like this when you have lot of account with alias but perhaps it can help someone who have only one like me