From the article:
Mastodon is a great microblogging platform, but maybe you want to try something new. Firefish made a splash recently, and a lot of people are talking about it. If you’ve played with it for a while and want to officially take the plunge, here’s some tips on moving from your Mastodon Instance to a Firefish one.
The screenshot at the start, for aliasing the old account to the new, says “#1 account you’re moving from” and has an “Add” button. I infer from that it is possible to direct multiple old accounts to one new one.
Ok, that will be handy, thanks!