Add quote.php to your /my-plugins/ directory.
Modify your post.php template to include the link, outputs "Quote" by default:
<?php bb_quote_link(); ?>
And modify post-form.php to include the quote text in textarea:
<textarea name="post_content" cols="50" rows="8" id="post_content" tabindex="3"><?php bb_quote_message(); ?></textarea>