It would be a good idea to have a forum only for phpbb3.
Please, excuse me for this question, but I do know anything about phobb3... Is there any way to each user has "their own account". In other terms, a forum for each client or "private forum"?
The idea is to share information (files, messages, etc) between my site and each customer (customer portal).
When you join the phpBB2 club you gain access, but the support is mostly for the styles from the club.
For general questions about phpBB3, you should visit
www.phpbb.com/
, take a look at the FAQs, browse or post in their forums, etc.
You can create user groups and permission levels, I don't know how many clients you have and if you would want to create a separate group / forum for each. You can communicate with private messages as you can do on this forum.
You may have to think of an additional / different system to do what you want.