add_action( 'appthemes_comments_form', 'your_function' );

<?php
// let's remove default ClassiPress blog comments form
remove_action( 'appthemes_comments_form', 'cp_main_comment_form' );
 
// use this comments form instead for blog posts
function insert_your_own_comments_form() { 
?>
    <div id="respond">
 
        <h2 class="dotted"><?php comment_form_title( __('Leave a Reply','appthemes'), __('Leave a Reply to %s','appthemes') ); ?></h2>
 
        <div class="cancel-comment-reply">
            <small><?php cancel_comment_reply_link(); ?></small>
        </div>
 
        <?php if ( get_option('comment_registration') && !is_user_logged_in() ) : ?>
 
            <p><?php printf( __("You must be <a href='%s'>logged in</a> to post a comment.", 'appthemes'), get_option('siteurl').'/wp-login.php?redirect_to='.urlencode( get_permalink() ) ); ?></p>
 
        <?php else : ?>
 
            <form action="<?php echo get_option('siteurl'); ?>/wp-comments-post.php" method="post" id="commentform" class="commentform">
 
                <fieldset class="form-comments">
 
                <?php if ( is_user_logged_in() ) : global $user_identity; ?>
 
                    <p><?php _e('Logged in as','appthemes'); ?> <a href="<?php echo get_option('siteurl'); ?>/wp-admin/profile.php"><?php echo $user_identity; ?></a>. <a href="<?php echo wp_logout_url(); ?>" title="<?php _e('Logout of this account','appthemes'); ?>"><?php _e('Logout','appthemes'); ?> &raquo;</a></p>
 
                <?php else : ?>
 
                    <?php 
                        $commenter = wp_get_current_commenter();
                        $req = get_option( 'require_name_email' ); 
                    ?>
 
                    <p class="comments">
                        <label for="author"><?php _e('Name','appthemes'); ?> <?php if ( $req ) _e('(required)','appthemes'); ?></label>
                        <input type="text" name="author" id="author" class="text required" value="<?php echo esc_attr( $commenter['comment_author'] ); ?>" size="22" tabindex="1" />
                    </p>
 
                    <div class="clr"></div>
 
                    <p class="comments">
                        <label for="email"><?php _e('Email (will not be visible)','appthemes'); ?> <?php if ( $req ) _e('(required)','appthemes'); ?></label>
                        <input type="text" name="email" id="email" class="text required email" value="<?php echo esc_attr(  $commenter['comment_author_email'] ); ?>" size="22" tabindex="2" />                                
                    </p>
 
                    <div class="clr"></div>
 
                   <p class="comments">
                        <label for="url"><?php _e('Website','appthemes'); ?></label>
                        <input type="text" name="url" id="url" class="text" value="<?php echo esc_attr( $commenter['comment_author_url'] ); ?>" size="22" tabindex="3" />
                    </p>
 
                   <div class="clr"></div>
 
                <?php endif; ?>
 
                <p class="comments-box">
                    <textarea name="comment" rows="" cols="" id="comment" class="required" tabindex="4"></textarea>
                </p>
 
                <div class="clr"></div>
 
                <p class="comments">
                    <input name="submit" type="submit" id="submit" tabindex="5" class="btn_orange" value="<?php _e('Leave a Reply','appthemes'); ?>" />
                    <input type="hidden" name="comment_post_ID" value="<?php echo $id; ?>" />
                </p>
 
                <?php comment_id_fields(); ?>
                <?php do_action( 'comment_form', $post->ID ); ?>
 
                 </fieldset>
 
            </form>
 
        <?php endif; // if logged in ?>
 
        <div class="clr"></div>
 
    </div> <!-- /respond -->
 
<?php
}
add_action( 'appthemes_comments_form', 'insert_your_own_comments_form' ); 
?>