Hello! This is an year old post.
You can directly import an animation from a skeleton to another, as long as the structure and the names of the bones match.
You still cannot copy keyframes from one skeleton to another, but you can copy bone transforms, meaning that you can select a bone, press ctrl+c
then select a bone in a different skeleton and paste all the transforms for the bone at the keyframe you copied.
What is it that you would like to achieve exactly? And why are you copying keyframes from a skeleton to another (to have a bit more of context)?