The default instance is your instance (local). In the setting you can change this to all. Under user settings.

There will likely eventually be a way to migrate your account just like mastodon, however kbin is a different platform altogether and you would need a new account there.

You need to register in the instance you want and then add the new account to the app

So can I log into Jerboa with my kbin.social account or not?
Edit: Turns out I can’t. Jerboa seems pretty useless to me if I can’t filter out the unwanted instances.