The BeOS is a new operating system, written from scratch to take advantage of the full power of multiple processors and new operating systems technologies. As a new operating system, the BeOS requires new applications be written for it. Macintosh applications will not run under the BeOS out of the box.
On PowerMac hardware (which provides the necessary Mac ROM image and license), you can run Mac OS applications in a "compatibility box" using the third-party application SheepShaver. You can read more about the product and its interesting name at the SheepShaver web site: