Not sure that's possible, hagai. You can mix & match images within the skin but can't toggle one set off & another set on within the same skin.
What exactly are you trying to do? Use a different set of trackbars for a specific application or (applications) & the normal ones for everything else, something like that? If so, you could create the new skin, change only the trackbars in it, and use it as a Per Application skin for the program(s) you want. That way all the programs would have the same skin appearance, except for the trackbars in the specific applications for which you choose to use the alternate skin.
Does that help?