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 :(
Oh man it didn’t take long for the reddit rage bait to arrive :D