I'm no Blender export, but isn't there an option to "impose" the shape of one mesh on another?
That would have been too easy :)
There is a Mesh data Transfer plugin for Blender, but it only works for characters with exact same UV set.
(Mostly for exchanging body parts between the same Base characters).
I have recently transformed 13 "G" characters to CC3+ base (https://forum.reallusion.com/527288/iClone-Nostalgy-Characters-Pack) and that was no fun...
Various techniques. Head-shot plug-in / sliders in CC. But mostly Blender sculpting to finish/refine details.
I have also bypassed Head-shot for some of them and used a Blender technique I have developed (for heads mostly) to import desired character morph slider into CC (nothing revolutionary though).
Here is a routine without too many details. Got to have some experience making sliders for CC from OBJ:
* In CC preliminary shape the head/body of the CC3+ Standard Male or Female. Symmetrise body mesh in Edit Mesh mode. Export OBJ > Nude character in Bind pose > Body only.
* Import OBJ to Blender and import OBJ of the original character you wish to replicate/transform into CC3+.
* Mask inner head parts, eyelashes, tear-duct, tongue and body for both characters. You should only leave outer head mesh with lips and have holes for eyes, nostrils and ears.
* Superpose them and try to sculpt CC character to match the Original (as close as possible). Most importantly you have to match holes and have 2 meshes pretty close near them.
In Edit use proportional edit mode with fairly large bias, so that masked inner parts do follow to some degree. There would be less fixing at the end this way.
* Use Shrinkwrap modifier on CC character (create vertex group for trimmed head-mesh and specify it under modifier) to wrap its head around the original character head. Result may vary depending how well you sculpt/edit mesh in previous step.
* Remove mask modifier and smooth the mesh of CC head with low strength smooth brush. Fix eyelashes, tear-duct and whatever else needs fixing.
* Optionally sculpt the body (you can also do selective shrink-wrap here too - but probably except fingers).
* Export CC as OBJ, import to CC as a slider and apply that slider to the Neutral base.
* Workout the texture with skin-gen.. or by any other means.