Welding has always caused problems in Zbrush, so I leave this option always unchecked.

So far the jacket is mapped and rigged, modelled for the standart M4 figure. Now I wanted to create a full body morph which I have done for other clothes millions of time.

My workflow is pretty simple: I load my morphed M4, conform the jacket, export both with the options "...body part names" and "existing groups in polygon groups" import in Zbrush with "triangles to quads" unchecked, create the morph in edit mode, export each group back to poser and load the targets there.