Sonic.net

how do I add extra email address?

Log into member tools at https://members.sonic.net/ Once you're logged in click on Email, then Manage Mailboxes. In the Manage Mailboxes section is where you add additional email addresses. Hope this helps!

But is there a way to add an alternate mail address as an alias for an existing mailbox, without
creating a whole new mailbox?

If you have your own domain and it's hosted by Sonic.net, yes. Otherwise, you just have your allocation of mailboxes.