Any 3D Suite will do this. You only need to assign the edited TGA file as the applied default texture......some even apply the new skin with Drag & Drop.
S3ditor will display the new texture ONLY after you have imported the altered texture into the DAT file, as this editor reads directly from the objects DAT file and ignores standalone textures. It's advisable to work with copy/backup DAT files.
Personally, I have a program called Deep Exploration which will do exactly what you need. The only caveats are that a) it's not free and b) you need to extract the model (OBJ) data from the DAT files to view anything.
The best/cheapest/simple option is to use S3ditor, this program will also allow you to locate and extract OBJ files (3d meshes).
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