Yes, but the 'ghost mechanics' of the Star Wars universe are different to those of the Harry Potter universe. Incidentally, they don't really do all that much with the ghosts in the HP films. I forget which HP book explains why some people become ghosts rather than passing on. In any case, it would cheapen the emotional impact of Dumbledore's death to have him come back as a ghost. Sidenote: I am hoping that John Williams comes back for the 6th instalment in the film franchise, just to hear what he would do with that whole sequence musically.