It's related to the geoset that robe uses. Meaning to edit that you would have to change the entire robe geoset, so every robe would be like that.That is model editing.
You have to edit the geoset, meaning the easiest way would be to convert the blood elf character model to m2i with m2mod3 and edit the geoset in blender 2.49there is only one geoset for the skirt bottom, i think, meaning you can only have one type of skirt model at a time.Doing this will remove the old skirt kind completely, and replace it with this.
It has to be blender because blender has the m2i plugin that allows you to convert and edit m2 files with animation.And no, to my knowledge it's impossible to make more than a single skirt geoset
it is the item that defines the texture on the geoset
I don't think it matters at all.You just go about what you have to edit.
Because there is only one skirt geoset, and you can't add any more geosets than blizzard has.What I think you could possibly do is replace the northrend legs geoset with a skirt geoset. I never tried that so I can't say if that can be done or not, so try and error.