OK, first off, before you export the character, make sure you apply the proxy topology that you want to base your new topology on--probably the "Adult Female Genitalia (new) (HEALED)", preferably with my UV remap (see
http://www.makehumancommunity.org/proxy ... ealed.html for instructions on making this change). You also want to set the sliders to full Caucasian and (in this case) full female; don't touch anything else (the armature shouldn't matter for this, but it's not gonna hurt anything, either). Then export through MHX2. From your description, the above is exactly what you've done (or close to it, depending on which proxy you used). (But joepal's method may also work--no one has tried it yet, so if it does, let us know!)
When you import into Blender, you should only need to import the proxy, not the base mesh. Once you've made your changes, save using MakeClothes, with the Average Female (and no helpers).
The one thing I don't understand is your remark about adding vertices: you can't add vertices to a proxy, by the nature of the beast, though I'm aware there's been at least one detailed discussion on how to get around this limitation.