i've never used Zbrush and don't have it (yet!) so i can't really advise on that.  i use Blender, and i know i can use it to make and export new polygroups.  i'd personally do it as a new material, and that way i know it wouldn't mess up the rigging.  so if i were you, i'd probably:

• import the original object into Zbrush
• select the new areas, make a new material zone,
• export the new object
• edit the cr2 with Cr2Editor to point at the new object (simpler than it sounds)
• save the new cr2 as a copy
• load the new version of the dress
• add the new material to the constrained group in the Cloth room

you don't even need to do the cr2 part if you're making the whole dress dynamic rather than a dynamic/conforming hybrid.