The issue can be fixed with
$db = FabrikWorker::getDbo(true);
in components\com_fabrik\views\import\view.html.php line 86
But working with multiple connections may have issues anyway (e.g. you can't do joins between lists with different connections, dbjoin elements to other databases etc).
And if the connection to the J! DB doesn't have id=1 and/or isn't the default - no idea what may happen somewhere else.
I didn't test but I'm pretty sure this frontend import doesn't do in Fabrik 3, too.