The plugin applies a bunch of (undocumented) filters on the different templates used for generating the output, which you might be able to use. All hooks are in Userlist.class.php (search for the use of the "apply_filters" function).
For your case you probably want to use the "aa_userlist_template" filter. You can use it by adding for example the following to your theme's functions.php:
<?php
function aa_userlist_add_link_to_all( $template ) {
$url_link = '/all-bloggers';
$url_text = 'View all bloggers';
$template .= '<span class="all_bloggers_link"><a href="' . $url . '">' . $text . '</a></span>';
return $template;
}
add_filter('aa_userlist_template', 'aa_userlist_add_link_to_all');
?>
I haven't tested it but this is how it should work in principle; you obviously need to adjust $url_link and $url_text, and add respective styling for span.all_bloggers_link to your theme...
The default value for $template is '<div class="author-list">{users}</div>' and above function simply adds a link to the end of it.
Hope this helps.
Ben