I don't believe this is possible w/ the Beolab 14
If I'm not mistaken (FWIW, I haven't seen many of these types of B&O devices), it has some sort of proprietary centralized connection module, not completely unlike what Bose uses. So in this regard, you would likely not be able to simply add on two more speakers to gain 7.1 sound.
Why not contact Beolab support directly since it directly involves configuration of their product line? You could also ask at AVS forums if they did not respond.