Stretchy body parts is not possible
To get someone's head on another ragdoll's body is to deflate all of their parts except the head, then no collide the ragdoll and deflate the head of a ragdoll that you want to replace and put the head on the body and freeze it, then weld them together, if they're the same ragdoll or have the same bone structure as each other (such as the Half Life 2 ragdolls), you can weld the arms to the other ragdoll's arms then do the same for the rest of the parts before deflating the parts
There may be problems when trying to move the ragdoll around though