Using action hooks inside of a shortcode

Try this:

function example_shortcode( $atts ) {

    $shortcode_output = "<p>Some shortcode content.</p>";
    $shortcode_output .= "<p>More shortcode content.</p>";

    ob_start();
        do_action('below_shortcode');
        $below_shortcode = ob_get_contents();
    ob_end_clean();

    $shortcode_output .= $below_shortcode

    return $shortcode_output;
}