Okay. Fantastico does not yet support custom packages, but the cPanelAddons does.
Your first step will be to modify any of the files you want. I suggest reading over the cpaddons documentation, just so you know what you will need to do in order to actually build and distribute the addon.
You can also use /usr/local/cpanel/cpaddons/cPanel/Bulletin_Boards/phpBB* as a starting point. The pm files hold the information that tells cPanel what it's supposed to do. The folder holds the actual files... the tar.gz is a complete copy of the board, without the install directory. You will need to manually edit the config.php file, to work with the autoinstaller. I'll post it at the end of this, so you have a copy. The mysql file is, obviously the SQL database settings. You're going to need to find the SQL for your own install, and edit it to look like this one... mostly the config table, and the members table (so that the admin account is properly created).
Once you have your modded board all set up and ready, tar -czf it up (use the filename to match the board version... so use something like 2.0.21.tar.gz), and upload it, your mysql file, and lisence file (this is the standard "we don't offer support for this" message) into it's own folder inside /usr/local/cpanel/cpaddons/cPanel/Bulletin_Boards/ (like, create a folder called phpBB_Modded, and upload into there). Now, create a file called <newfolder>.pm (replace <newfodler> with the name you uploaded the files to), and essentially edit the phpbb.pm file to match it. You'll want to change the version information, addopt_url, description, and anything else to customize this as your own build. Make sure the file is located in the same directory as the other phpBB.pm file and your folder. Do not use the same name as phpBB, and do not overwrite the existing one.
The cpanneladdons documentation will walk you through on how to distribute the build if you are wanting to... this will also tell you how to enable or disable the addon for your customers.
In all honesty though, this is a lot more complex than you are needing, since you would still have to log into cPanel and create the board. What you are wanting might be better handled with a program like
http://www.nbsdesignz.com/Multi-Forums_phpBB. But anyway, whatever you chose...
Code:
<?php
// phpBB 2.x auto-generated config file
// Do not change anything in this file!
$dbms = '[% phpbb2_dbms %]';
$dbhost = '[% mysql.phpbb.sqlhost %]';
$dbname = '[% mysql.phpbb.sqldb %]';
$dbuser = '[% mysql.phpbb.sqluser %]';
$dbpasswd = '[% mysql.phpbb.sqlpass %]';
$table_prefix = '[% table_prefix %]_';
define('PHPBB_INSTALLED', true);
?>