I think that this one works,haven't done it for a while, so not sure :

[1]
Load original figure from the Geometries folder, uncheck everything except "make polygon normals consistent"
Grouping tool
Spawn Props
Export head and neck with all object export options without "save as morph target"name this something like Head&NeckGrouped
New document
Load Head&NeckGrouped, uncheck everything except "make polygon normals consistent"
Now load in this "prop" your custom Zbrush morph
Grouping tool
Spawn Props
Export Head as morph target
Export Neck as different morph target

[2]
Also if you exported with groups at first place
Load your custom Zbrush morph, uncheck everything except "make polygon normals consistent"
Grouping tool
Spawn Props
Export Head as morph target
Export Neck as different morph target

My FreeStuff