Your question got me interested, so I decided to do a little experimentation. Unfortunately you can't (as far as I know) animate the MatSpheres (plural). You can animate one MatSphere as long as you are prepaired to leave the Joint Editor open, but one does not sound like it would be much use to you. Never the less, out of academic intrest here is how to animate one MatSphere. Using a text editor create a valuParm channel in the BODY actor, eg:

           valueParm InnerMAT X
                        {
                        name InnerMAT X
                        initValue 0
                        hidden 0
                        forceLimits 0
                        min -100000
                        max 100000
                        trackingScale 0.001
                        keys
                                {
                                static  0
                                k  0  0
                                }
                        interpStyleLocked 0
                        }

Or you can use an existing valuParm if the figure contains spare ones you dont need to use for other purposes. Next you need to create a pz2 to inject slaving code into the figure: {

version
        {
        number 4.01
        }

actor innerMatSphere
        {
        channels
                {
                translateX xtran
                        {
                        valueOpDeltaAdd
                                _NO_FIG_
                                BODY
                                InnerMAT X
                        deltaAddDelta 1.000000
                        }
                }
        }
figure
        {
        }
}

In Poser, load the figure, select the actor you want to animate, open the Joint editor, select the paramiter, select the matSphere, apply the pz2. You can now select the BODY and enter diffrent values for the InnerMAT X dial in each frame of the animation. Running the animation will cause the innerMatSphere to respond to changes in the master InnerMAT X channel, providing that the Joint editor is kept open throughout the whole operation. It seems (and this is just supposition, but seems to be borne out by my experiments) that when the Joint Editor is open and an innerMatSphere selected, that Poser creates a tempory actor "actor innerMatSphere", and this actor is suseptable to ERC controle the same as any other actor, but as soon as the Joint Editor is closed the tempory actor disapears. Because only one innerMatSphere actor can exist at one time, and this actor is somehow associated with the currently selected actor, only one innerMatSphere can be animated. It's yet to be seen if both the inner and outer sphers can be annimated at the same time. In the above example only xTran was animated, but it should be posible to animate any and all of the MatSpheres translate, rotate and scale channels, but only for the currently selected actor, and only whilst the Joint Editor is open. Like I said, none of this is likely to be of more than academic interest to you, but then who knows, it may help someone somewhere down the line with a diffrent problem. P.S. The experiments were done in P4.