Is there a way to import already weight painted clothing or other wearables into Character Creator?
The workflow would look something like this:
1. Export base character and rig from CC3.
2. Import them into Blender, via whatever method that preserves the deform bone names.
3. Conform CC3 character to existing character in Blender via whatever method, such as Shrinkwrap modifier, or project geometry in Zbrush and bring back to Blender.
4. Delete or hide existing Blender character, so only modified CC3 base is visible.
5. Position clothing and weight paint it in Blender, assigning to CC3 character bones.
6. Export clothing from Blender in some format, maybe FBX.
7. Import clothing into CC3, preserving vertex weights done in Blender, so they do not have to be redone in CC3.

I am yet unfamiliar with 3DXChange, so maybe it would be helpful with some of these steps.