There’s no real embedding of another software into wordpress. If you want to visually embed it, then you’d have to take a look if and how phpBB uses templates and then add the same nav to the phpBB templates. The look and feel can be done via css. You’ll have to
- a) add the same css IDs and classes to the phpBB template parts as you got in your wp templates or
- b) Make a new stylesheet for phpBB and asign the same style definitions you got in your wp stylesheets to the according phpBB html-elements.