I think that is easier to do in Poser than in DS. You load both figures and then use the Hierarchy editor to turn of those parts of each that you don’t want to see, and parent the part that is left to the other figure, so in this case turn off all but the body and neck of the dragon and the other parts you want., and then parent the head to the body of the other figure, which has the head turned off, or maybe parent the body of the other figure to the dragon. There needs to be scaling as well, so the figures are the same size.
It is still not really that easy, but it can be done.
I have transplanted heads in the past, but usually simply the head. I made a Female “wolff” that way, wearing a dress.