You can parent a prop to any part of a figure. Fore xample a ring would be parented to the finger you wanted. Earings to the head, etc.

If you load a prop that was smart proped for a different figure, for example a sword parented to M3's hand loaded onto Hiro. The sword will still be parented to the hand but you will most likely have to adjust it's position a bit, in order to compensate for differences in the characters' sizes.