I am trying to figure out if this is possible. If it was it would lower the file size for detailed models with morph targets.
So far my tests have failed and i can find no examples of this.
As can be determined from the THREE.SubdivisionModifier's source, it does not support morph targets. So you'd need to patch it to make it work. To lower file size of animated objects, using skeletal animation is an option to consider.