Hi gorzek,
It is all stored in $_SESSION, however, by default, xataface stores its session data in its own folder so that it can have complete control over garbage collection etc...
By default, Xataface will store all of its session files in a directory named 'dataface', a subdirectory of the default session save directory.
E.g. if your normal session data directory is /tmp, then xataface will store its files in a directory named '/tmp/dataface'.
You can follow the logic of how this is in the startSession() method of the Dataface_Application class (Dataface/Application.php).
One strategy that could work for you is to set the session_save_path() for PHPBB to use the /tmp/dataface directory (see
http://php.net/session_save_path), or modify some code in Xataface to prevent it from changing the directory (really this should be configurable, but it is not currently).
Once you are using the same session data, merging login info is easy. Xataface just checks the $_SESSION['UserName'] variable. If this is set, it assumes that a user by that name is successfully logged in.
-Steve