“Read more” for the_excerpt()

I’m trying to port an existing blog from Textpattern to WordPress. In Textpattern, I was able to say, if there’s an excerpt use the excerpt and show a “read more” link at the end. Otherwise, show the content field. In WordPress, I get most of the way there with this:

<?php
if($post->post_excerpt){
  the_excerpt();
} else {
  the_content('read more');
}
?>

This allows me to choose between using the excerpt and the ‘more’ tag in the content field. The problem is that I can’t figure out how to add a “read more” link to the excerpt since it doesn’t allow parameters. FYI: I can read and understand PHP, but can’t write much myself, yet. Does anyone have any ideas?