So you mean the opposite of external.php of vB3? Something that will grab xml feeds from sites and puts them on your forum?
See, that's a bit tricky. It's quite easy to do, but such things are generally done via other means. There are XML grabber scripts for sites, but for putting it on a forum.. that requires forum-side caching and periodical grabbing. Lot of effort for something that's not very important (imo). Most people who use xml feeds use them in their aggregators/programs, not on their forums.