url rewrite .htaccess extension to permalink

This should work for you-

function wpse28906_rewrites_init(){
    add_rewrite_rule(
        'mypage/([^/]+)/?$',
        'index.php?pagename=mypage&myslug=$matches[1]',
        'top' );
}
add_action( 'init', 'wpse28906_rewrites_init' );

function wpse28906_query_vars( $query_vars ){
    $query_vars[] = 'myslug';
    return $query_vars;
}
add_filter( 'query_vars', 'wpse28906_query_vars' );

You can then use get_query_var( 'myslug' ) in your template to get your slug value. Don’t forget to flush your rewrite rules after adding this by visiting the permalinks settings page.