actually you can add content to the bottom of your phpbb3 forum manualy.
To do that :
find the overall_footer.html (inside theme/template) file and open it with your fav text editor. Inside you should be able to add as many code as you want....
Make sure to empty the cache of your fourms from the admin area and refresh the templates as well.
As for a home page you will need to have it at a different path. The main page of the forums is called index.php so if you modify it you kill the forum.
have it like :
www.yoursite.com/index.html (wich is your site home page)
www.yoursite.com/forum/index.php (forum home page)
the forum is in the forum folder. the index.html and index.php are separated.
Also, there is actually a phpbb3 portal, i used it... well i admit its pretty cool but in my time (6 or 7 month ago) when RC1 of phpbb3 came out until RC4 (where i stoped updating my phpbb3) the portal was pretty buggy and had still lots of work before being able to be used correctly. I made my own with help of some php masters, tweaked some codes and was able to come up with a site .. fully working site based on the phpbb3.. pretty good since now everything is "protected" and "controled" via the phpbb3 admin interface