I can have milk for my hexa (which doesn't really work for me) but if I put that same milk in a yoghurt maker with some probiotic bacteria and turn it into natural yoghurt it is no longer a hexa - why on earth not???
The healthy extras are only suggested as compulsory so people on the plan get a certain amount of fibre and calcium, if you're getting enough calcium for your needs through that yoghurt and that works for you I don't see any problem.