I think the problem might be exporting both neck and head.  Those are clearly separate body parts in Poser and chances are that importing them into ZBrush may have welded the two meshes.  This may explain Target Geometry error.  I couldn't be sure, and someone else may need to verify this for me.  Nevertheless, when importing target morphs back into Poser, you can only select and import one mesh at a time.  For example, you smoothed wrinkles from around the eyes; in this case, select only the head in Poser, export as OBJ, create your morphs in the 3rd party app, then import the target morph to only your model's head in Poser.  You should be able to apply the target morph to either fresh or old characters.