Soon we'll be able to recognize kbin users from their jacked index fingers

Your mastodon account is only good for loggin in the specific instance of mastodon where you made the account.
As for posting into kbin from mastodon, you can follow a magazine as if it was a user, for example i'm, @Mastur, whereas this magazine is @fediverse. That's what i've understood so far, maybe if the federation was working properly i could test/try stuff further :(