Convert PRT to SKEL

No direct option. 

Check article CS51562  for alternate options. 

Without maintenance I cannot read that support article, so my apologies if this is a duplicate of that.

You can only create a skeleton in the context of an assembly. This is from Creo 2, but later versions should be similar.

1. In your target assy, select "Create" from the component area of the Model tab.
2. Select "Skeleton Model" in the "Component Create" dialog.  Enter an appropriate name and select OK.
3. In the "Creation Options" dialog select "Copy from existing".
4. Click "Browse" and navigate to the part that you want to convert to a skeleton and select it.
5. Click OK in the "Creation Options" dialog.

You now have a skeleton that is a copy of your source part.  If you already have parts that copy geometry from the original part, you will have to create new references to the skeleton instead and fix any children. 

You may have luck (if you are not using Windchill) in renaming the new skeleton part to have the name of the original part and therefore "tricking" Creo into using the skeleton instead, but that requires some planning and managing what is in session when you do the rename within Creo (don't do it in Windows).