Add the sub forums as links. Create a subforum and make it link to the other forum. I have done this on my site but made a link to another site as a subforum. Works great. The only thing is the subforum icon doesnt light up with new posts and it is the link subforum Icon. If that bothers you and you only use that subforum link as a subforum. You could rename a copy of the subforum icon to the name of the link subforum icon and presto, they will match. It wont light up with new posts but it will match.
Hope that helps...