Unfortunately there is no easy way to achieve this. As you stated, CMAttitude
doesn't provide any features to manipulate it or to create an instance. That said you have to do the multiplication on your own. Furthermore you will need an additional class for quaternions because CMQuaternion is only a struct.
The good news is that all the code you need is available already in the cocoamath project for example. Take a look at How multiplyByInverseOfAttitude (CMAttitude Class ) is implemented? for some guidance and more links to the basics of quaternion math.