Kylo sadly it's not using an ItemList so it's not super pretty but something like this should work (untested):
import {extend} from 'flarum/extend';
import TagSettingsModal from 'flarum/tags/components/TagSettingsModal';
// Your intializer .add function
extend(TagSettingsModal.prototype, 'form', vdom => {
vdom.push(m('.Form-group', /* your stuff */));
});
// End of the initializer function