Look at the BTB help file. Go to the section "Editing \ Materials & Textures". Adding new materials is explained in there. When you copy a material, all the files needed for the original will stay in the XPack's zip file, and the files for the new material will be in a folder in My Projects\"project name"\XPacks. If you copy a material with diffuse and bump textures, a new XML file will be created (in the new folder) along with a little jpg preview image. If you load another texture, the image file gets copied to My Projects\"project name"\XPacks\Common\Textures. If you only change the diffuse texture, the bump texture will still be read from the original XPack.
Must dash. Try to muddle through...