Well .. you open up ItemDisplayInfo.dbc now.
Then you go to Allakhazam Item Search and search for your item. In this case your t4 kilt.
After you got that, you go to Links: AH Pricing Details, Item History, XML, Lite, TXT below the item tooltip and serach in that XML file for "DisplayInfo" then. You've got a number like 28728 then.
You switch back to your DBC and go to that row. In my example with 28728 its:
28728,"","","","","INV_Pants_13",0,0,1,0,0,7,0,0,"","","","","","Robe_Horde_B_05 Purple_Pant_LU","Robe_Horde_B_05Purple_Pant_LL","",0,
You see: ID, model1, 2, 3, 4, Iconname, geoset1, 2, 3, 4, unk, groupsound, helmet vis data 1, 2, texture 1, 2, 3, 4 ... etc, itemvisuals. What is interesting to us is the geoset part. If you search for a leggings you get an entry like:
25833,"","","","","INV_Pants_04",0,0,0,0,0,11,0,0,"","","","","","Plate_A_01Silv er_Pant_LU","Plate_A_01Silver_Pant_LL","",0,
You see at the geosets: The leggings got 0,0,0,0 while a kilt has 0,0,1,0. Well that's all. Just change that field to 1. Its the 3rd after the Iconname. But I think you should find it if you search for an kilt and trousers to compare the entries.
I hope that's it.