Apply filter to output?

Is there a way to apply a filter to the HTML output of the widget, that we can place in our functions.php file so updating won’t overwrite it?

Looking to do the equivalent of:

		$html .= '</div><!-- Generated by http://ww.wp.xz.cn/plugins/recent-posts-widget-extended/ -->';

		$html .= apply_filters( 'myfilter', $html );

If I posted that in the plugin functions.php file itself.

So, any way to do that, but keep it in the theme functions.php file.

Thanks!