open the skeleton character and import it into the same scene as your mummy, move him over the mummy, and then hide/remove the mummies arms and replace it with the characters arms. You may need to edit the joint parameters and such. Look at some of the tutorials her for replacing body parts or creating new poser figures. Its possible and not terribly difficult, but your workload just tripled if you plan on doing this. Its great practice though :) And also the best start you can get for creating yor own fantasy characters.