That’s not possible with BuddyPress at this time. See this: https://codex.buddypress.org/getting-started/faqs/
In fact, I don’t see this ever being an option in BP–even when the new bbPress plugin becomes available. Both BP and the upcoming plugin version of bbP are software layers that sit on top of WordPress. So each is responsible for their own sets of data access functions. Even with BP Group-based forums, the bbP functions are being hijacked to work within BP but only barely.
To have a standalone install of bbPress functioning in WordPress (which is what would give you a site-wide forum) requires a process called deep integration. For more information on integrating bbPress with WordPress, search the WP or bbP forums. Or better yet, just wait until the pluginitized version of bbPress is available!
