(1) This isn't a Unity-specific question. I'll be moving this to the Editor forum soon so it can benfit other users. Let's open a new topic if something Unity-specific comes up. In any case, this question involves many smaller problems. Some of which may have already been answered.
(2) There are many possible solutions for what you want.
I can offer my answer, but I've never implemented something like this for production before so others who have actually made the whole pipeline work in their games may have better and more detailed answers.
In general, you have to add all the attachments to the skeleton. The robot head. The animal head. The skeleton head. Put them all in Spine in one skeleton. Animate normally.
And let the runtime handle changing the head, or the legs, etc. You can do this by dynamically generating a skin object for the skeleton to use (which stores what head or leg to use).
mcplata wroteThe problem is how to define the skeleton data and json dinamically in order to allow the user to choose the parts and then generate the data and jason for the squeleton... Its this possible?
You seem to be preempting a solution of your own. This (regenerating a new json and reloading that) is not necessarily a good way to do it, but if you understand how Spine works and it makes sense to you with your understanding, go ahead.