Very custom search

Hello there!

I have to make a custom search for a wordpress site. This site has posts, pages, custom posts and custom pages. Searching in these kind of contents is easy task, but I should extend the search in a more difficult way. The wordpress should search for the given string not only in the contents mentioned above, but in a xml file. So if the user search for ‘foo’, the search result must contain not only pages, posts with word ‘foo’ in it, but the xml hits too. Working with xml by simple xml is easy task too. But hook together the wordpress bult-in search and my xml content is too hard to be solved just by myself. So any little useful advice welcome here lads! Thx a lot!