We do not recommend or support connecting alias accounts. You can only add it as a separate account like you would with a new email.
Since aliases use the same sending account behind the scenes, it gets exposed to your provider (such as GSuite), and they can flag your sender's reputation which would negatively impact your deliverability.
The email accounts that you're connecting to Mailead need to be real users with paid user licenses and real inboxes.
If you try adding a Google alias account, you will see an error message