Ok, I think I can help with this one. First you need WoD character models for 3.3.5. They are the same models used in BFA. Next use photoshop script from downloads part of this site, that converts skin textures to 3.3.5 layout. Save them in correct blp format (indexed). Then create transparent .blp for underwear, name them accordingly and make sure they are also in an indexed format with an alpha channel. Then you're done, build an mpq file and you are ready to go.
P.S: also make sure to remove "HD" from texure names.