Well you could crawl all the internal and external the same time. Or you could just do site:domain.com on bing and tear thru it and then load up and extract external. Or you could keep doing what your doing. But doing both internal and external at the same time makes sense. Then after each run just remove urls that are not part of the original set of domains and then keep going. By the time you get done, on your last round you will just do external and then your set. I think the page scanner addon is what your looking for. You can use multiple words, RegEx and also like the AND operator.