BuddyPress.org

If you are asking if can you use some other forum software instead of bbPress as a sitewide forum, then the answer is yes. If you are asking if can you use some other forum software instead of bbPress that will share user credentials or function as a group-based forum component, then the answer is no–not without doing a lot of custom coding.

However, I believe that Simple:Press does share WP user credentials, but I am not positive about this as I do not use it.