If they’re in the same DB, it’s easy enough. Pull them straight out of the DB using $wpdb. And there might even be a WP plugin or a BB plugin that does this for you.
If they’re not in the same DB, but on the same server, you can access them by prefixing the DB table with the DB’s name, e.g. mybbpressdb.bb_posts, or something to that effect.
If they’re not on the same server, you need to create a new connection, using mysql_connect(), mysqli_connect() or a PDO instance. If you use either of the first two, be sure to use the parameter that makes mysql create a new connection — else it may discard the one from $wpdb.