Are you talking about making a decal as opposed to a morph slider?
there is this misconception that you use polygons to add detail which is true to when sculpting but you always lower polygon count on final product especially in a game engine setup.
so the morphs are for general shape/size
to add actual fine detail you bake the detail from your zbrush sculpt into a normal map and put it on the lower poly CC character. For morphs you can't change vertex information the order/number has to be identical. I believe there is a paper on this somewhere on the realussion site.. if i find it i will post the link, but your detail goes on a normal map not on the polygons of the character those just are there to hold the texture information which includes normal maps baked from hi poly sculpts
edit here is one link to tutorial information there is one for zbrush on there, but the detail you speak of is baked out onto the normal map. https://www.reallusion.com/character-creator/tutorial.html
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