Existing install in subdirectory, need to now include root with html pages

I’ve got an existing WP install in a subdirectory, with several hundred posts, and I want to now convert the root of my site to wordpress as well, so I can manage the appearance easily across the whole site.
At the moment, I have about 40 html pages in the root (and another subdirectory with a phpbb install). How can I add the root pages, keep all the same URLs (don’t want to lose SEO) and keep the blog in it’s original subdirectory? I’ve tried a duplicate install in the root, with adjusted htaccess to handle the permalinks, but it seems an unnecessary duplication.

Here’s the site if needed:
http://www.brighteyecounselling.co.uk/alcohol-drugs/

Thanks in advance.