You can load the hand pose and then save it out with the rest of the full body pose and it should save fine and you should be able to reload the entire pose from the library. I've never heard of hand poses not saving with full body poses unless there's a problem with a specific figure.

It may be an issue with the pose dials. If the hand poses were created using the pose control dials, then I'm not sure that those dial settings will get saved with the rest of the pose. The pose file may need to be edited manually for that.