I haven't tried this but I think you could make a series of bones that zigzag up the length of the accordian(start from the bottom and build the zigzag skeleton up (Y direction)). Then, try hooking up an IK from the bottom joint to the top joint. If it works, you should be able to grab the IK handle and stretch and bend the skeleton. Now for the hard part; you have to weight the accordian to the zigzag skeleton.

Like I said, I never tried this but it's all I've been able to come up with....