I've been creating some morph sliders in zBrush as I learn, and wanted to make something a bit more detailed.

Previously just using the move and smooth tools and had no issues and created some modest looking sliders.

This time I wanted to try making something detailed so I created several subdivisions and modeled a highpoly object onto the base face. It looks really good and I am pleased how it turned out, but when I try to import it as a slider like I have so many times before I get the error message "The base and target characters are incompatible".

If I lower the resolution all the way down it will import as a slider, but you can't even tell what it is.

I'm guessing that CC compares the poly count in the key or something. Is there a way to import it at all without loosing all the detail?

Is there a way to import it at all without loosing all the detail?

you can bake some details in a normalmap 

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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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Sorry for the late reply, but thank you for your response.
It seems the only way to reproduce fine detail is with decals then, and anything else would have to be modeled by hand and baked into a normal map.

...no not decals those are for on top of the textures usually its via normal maps which are bump maps that is how you get fine detail transfered onto a lower poly model from a sculpt and get it into a realtime engine. 

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"You have brains in your head. You have feet in your shoes. You can steer yourself in any direction you choose. You're on your own, and you know what you know. And you are the guy who'll decide where to go."
                                                                                                                                                                                                                                                                                     Dr. Seuss