Ok guys, it is EXTREMELY easy to update the old uMOD-based mods into the gpk file using the new tool. Not sure why gusset insists it will be a lot of work, especially since he did most of the heavy lifting extracting and editing the textures already. Basically, the uMOD packages contain custom nvidia textures (.dds files) that are loaded into the game on-the-fly by uMOD. All you need to do is open the original .gpk file, locate the appropriate texture within, import the correct .dds file from the uMOD package, and save the .gpk package.
Step by step:
1. Extract the uMOD package archive and open the .dds files to look at the image texture (Windows Texture Viewer is a nice and light tool for this)
2. Open the appropriate .gpk file for the outfit you want to edit. Basic filename structure is [race]_[gender]_[H/L/R].gpk. H/L/R = Heavy/Leather/Robe armors. Some of the later armors have their own gpk files, but it should be obvious (like Aman_F_L19.gpk is the leather armor model 19)
3. Locate the appropriate texture file within the gpk package using the TERA package editor. Right-click and select "import", then browse to the appropriate .dds file from the uMOD package. You can match texture files just by visually looking at them. The TERA package editor doesn't show the whole texture image for large textures, but you can just export it and look at the texture using WTV.
Now that we can load custom textures we can have texture editors work their magic to make custom edits.