You have mailboxes at https://my.webfaction.com/mailboxes. Those are not email addresses.
What do you see at https://my.webfaction.com/emails where your email addresses are managed?
If you see nothing there then you have no addresses to migrate.
The migrator only migrates mailboxes that are attached to addresses.
If you don't have any addresses then your mailboxes can't be migrated with the migrator.
If you want us to load your mailboxes manually then get your mail archive from WF and upload it to your shell user home directory on Opalstack and then email support to let use know the shell user name and location of the archive.
If you want to create email addresses with a script then you can use the mail/add API endpoint, eg:
curl -X POST \
--header "Content-Type:application/json" \
--header "Authorization: Token YOUR_API_TOKEN_HERE" \
-d '{ "source": "user@domain.com", "destinations": [ "UUID_OF_MAILUSER1", "UUID_OF_MAILUSER2" ], "forwards": [ "user@otherdomain.com","user2@otherdomain.com" ] }' \
https://my.opalstack.com/api/v0/mailuser/add/
