non WordPress files in Sitemap.XML

I’m using the Google XML Sitemaps plugin which works really well. My situation is a little bit trickier though and I could use some advice. We have a blog that runs on WordPress but it’s a separate installation that’s installed under domain.com/blog, we also have a phpbb forum that’s found at domain.com/phpbb.

Is there a way to get these directories onto the sitemap.xml without having to add every single page individually? The sitemap plugins allows to to add additional pages, but it only allows you to add one page at a time.

SEO is very important to this site, so having all the pages from the blog and forum on the sitemap is crucial.