Allows rounded borders, drop shadows and other CSS3 styles to work in Internet Explorer 6-9
Almost nothing! All you need to do is view the WP PIE page in the WordPress admin panel and the plugin does the rest.
In simple terms, you can't. Having said that, if you're reasonably comfortable with PHP you can edit the $stylesheets array in wp-pie.php to include the stylesheets you need and it should function in the same manner.