How low poly do you want to go? It might be easier to just use G3 characters (which should be lower poly count than CC characters) but then you lose the CC customization functionality. Save as an iAvatar It is doable but a lot of work. Import the FBX character file into 3DXChange and it should recognize it as a character. Ensure that the selected program supports FBX files and preserves the FBX armature during the ploy count reduction process. Open the FBX file (character mesh) in a program that will decrease the poly count.
You can get the Armature (FBX file) from the CC Templates which are provided for making CC Cloth. This is a similar process as used for making CCloth (form which there is a tutorial on the forum). Using free software like Blender, re-attach the reduced character mesh to the CC armature (bone structure). Use the program to reduce the poly count and save the reduced OBJ character mesh. There are a few free or trial software that will do this. Open the OBJ file (character mesh) in a program that will decrease the poly count. Then use the Export Mesh in Replace Mesh tab of 3DXChange to export the character to OBJ file.
If you can't do this directly from CC then push to 3DXChange (save/load or push to iClone and then push to 3DXChange). (Please note that I did not try this process so there might be some slight variations needed but the general process should be correct) 1.
ICLONE 4 FREEWARE PRO
If you only have Standard or Pro version then you cannot export FBX, so you export the OBJ mesh, reduce the poly count but then have to re-attach the mesh to the iClone armature. Basically with Pipeline you can export as FBX so, if you can find a poly count reducing program that supports FBX, the process is much simpler. The work flow would depend on if you have 3DXChange Standard/Pro or Pipeline.