I don't know about copying a partial pose, but you can save a partial pose to your pose library and then add it to the figure in the next frame.

Just open the Pose library. Click on the + sign at the bottom. In the window that comes up  select the parts of the body you want to include for the pose. It will ask you if you want to save the morphs, say no because you only want the pose, not the character morphs saved.

For example if you want to save an arm pose, select the parts of the body  from the collar to the fingers of the desired arm or arms. If it's only hand poses, save it with the hand and fingers selected.  Again, don't forget to cancel the "save morpns" portion when asked :)

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi