add static xml sitemap

Hi,
Is there a way to add/hook a static xml sitemap (generated with a crawler) to the sitemap.xml that is generated automaticaly with a custom code in functions.php ? I’ve got a lot of pages that doesn’t get updated very often, so I don’t need SEO press to dynamically update all the time on this. It would be great to have this option somewhere in sitemap configuration it should be an input box where I would put my xml link. What do you think of the idea ?