function my_rewrite_rules_array( $rules ) {

    $rules = array_merge( array(
        'foo/bar/?$' => 'index.php?pagename=foo/bar',
        'foo/baz/?$' => 'index.php?pagename=foo/baz'
    ), $rules );

    return $rules;

}
add_filter( 'rewrite_rules_array', 'my_rewrite_rules_array' );