You and I ended up with the same problem -Polygroups and Reallusion's Character Creator are not compatible -not yet at least.
I have a few solutions.
1. always make a back up both in CC3 and if you need to revert to Zbrush quick save - I know, just stating the obvious first.
2. Ok *A* SOLUTION. You will need to test this out your broken CC3 character before continuing this workflow. The method is that after you've accidentally polygrouped something in iClone and basically trashed your model
you will need to try the following: send your character back to iclone and export it as an obj set to full body or what ever your case may be. Then from the CC3 character base you started with load it into the scene. Once loaded create a morph slider --->Create, New Morph Slider. Give your new slider a name, select "current morph" in the source and with target morph set to "File" then choose your original trashed Obj you exported. Also choose "adjust bones to fit morph" and if you want to see results right away choose "auto apply to character".
If you painted anything in Zbrush with the broken CC3 Character the textures mostly likely wont fit the model but any painting and texture exports should work with your new morphed character brought into Zbrush as long as you don't use polygroup or change the UVs.
Hope that helps.