Just adding the geoset in doesn't assign it to any animation bones, so you need to do that manually.
Assign the vertexes of the geoset to the specific bones that control animation for that part.
You can do it in mdlvis, no I'm not posting how to do it specifically... because I'm busy, but here's the basic idea:
Open the model in the sequence editor of mdlvis, select the bone (green square) that controls the movement of the area around your new geoset.
Attach the vertices of the new geoset to that bone (Select the bone and then select the vertices at the same time, then click the assign button).
If you did it correctly the new geoset should follow the movement of the bone.
You can also use the mdlvis sequence editor to change how things are animated on each key frame. (Key frames are the little blue lines in the bar of white, they tell the bones where to move, changing bone position on a key frame will make the model move toward the new bone position until it queues the next key frame)