you trying it in blender? I do everything through that.
In blender you would first open up Zara. Go to edit mode and then go to the vertices and select the torso, biceps, clavicles, and then forearms. then click mesh mark seams, and then separate it. Save as a blend.
Do the same to FDR but delete everything but the suit, then export as a nif. go back to your zara.blend. import the fdr suit. line it up and size. go to edit mode select all. then go back to object mode and join the objects. go to edit mode and assign the whole highlighted suit to the torso.
then select al the parts of both hands, including fingers. separate and the press xto get rid of them. export as a nif.
NIFSkope
open your zarafdr nif and open a copy of the original zara nif. remove the original zara branches and then copy the branches of your new zara and paste the branches in the same ninode your original zara trishapes were.
thats how i do it. sorry not too much detail but its late. my blender guide has more details. see my sig
This is maybe a three hour job at most (well once you get used to blender)