Define “easy.”
A quick and dirty php way of working this — could probably do something with javascript, but for that I’d have to do some looking up — add the following code block into the comment form textarea tag (<textarea> <em>here</em> </textarea>) of the comments.php template for your theme:
<?php
if(isset($_GET['reply-quote'])) :
$reply_quote = $_GET['reply-quote'];
$quote_comment = $wpdb->get_row("SELECT comment_author, comment_content FROM $wpdb->comments WHERE comment_ID = $reply_quote LIMIT 1");
echo "<blockquote>\n<cite>" .
$quote_comment->comment_author .
" said:</cite>\n" .
apply_filters('get_comment_text', $quote_comment->comment_content) .
"\n</blockquote>\n\n";
endif;
?>
And in an appropriate place in the comment loop, if you use default permalinks (/?p=#) on your blog, add this:
<a href="?<?php echo $_SERVER['QUERY_STRING']; ?>&reply-quote=<?php comment_ID() ?>#postcomment">quote in comment</a>
If you use custom permalinks, go with:
<a href="?reply-quote=<?php comment_ID() ?>#postcomment">quote in comment</a>
