Automatically add to category

Hey Guys.

My goal here is to do a kind of “catch all”, where if a post meets certain criteria, it would be put in a specific category automatically. For example, if the post has a Youtube Video, it would automatically be put into the (pre-existing) category, “Videos”.

<?php
	$youTubeVideo = get_post_meta($post->ID, 'Youtube Video', true);
	if (file($youTubeVideo) && !in_category('33387')) {
		// It has the Youtube meta but it's not in Videos.. so put it in Videos.
	}
?>