<?php

add_action( 'add_meta_boxes_comment', 'my_comments_more_info_metabox' );

function my_comments_more_info_metabox() {
	add_meta_box( 'my_comments_more_info', __( 'Additional Info' ), 'my_comments_more_info_metabox_display', 'comment' );
}

function my_comments_more_info_metabox_display( $comment ) {
	echo 'User Agent: ' . esc_html( $comment->comment_agent );
}