It is not possible to directly add ACF field groups to the widget page. Basically it is not possible to add ACF field groups to any of the WP admin pages. Honestly, I don’t know if it is even possible to alter the widgets page or any WP settings page at all without making changes to WP core. I just did a quick search on this and came up empty.
If there were any hooks that let these pages be modified I’m sure that ACF would already be doing it.
