Adding a custom line of text to php code

In the insert_posts function, before the line $post_id = wp_insert_post( $post ); add the following (and suit to your needs):

Link to Twitter User Page

$post['post_content'] .= '<a href="http://twitter.com/__USERNAME__">'
    .'Follow me on Twitter'
    .'</a>';

and/or

Link to Twitter Status

$post['post_content'] .= '<a href="'.$post['twitter_permalink'].'">'
    .'View this post on Twitter'
    .'</a>';