Okay, I think I've got it.
First I have to check out that the *.obj file has its texture in *.jpg, *.tga or *.png.
Then I have to copy the files into the ufoaisrc folder under base/models.
Then I have to open UFORadiant.
There I have to go to entities & doubleclick misc_model & wait.
Now I can choose my new model, which should be displayed with the correct texture.
After clicking ok, I have the new model in UFORadiant.
Now I can save the new model as *.map & can copy this *.map file into UFORadiant-1.5/prefabs.
After Relaunching UFORadiant I have a new prefab
Some problems still remain:
1. How can I check if the prefab has the correct texture in UFORadiant, the preview window only shows the wireframe ?
2. Some objects seem to get holes here & there when applying the texture in misc_model !?
If I have a model as *.md2 & *.obj with a texture in *.jpg or *.tga, in misc_model only the *.obj will get textured, but sometimes with errors (holes), but the *.md2 will be displayed correctly, but without the texture ?! How can I correct this or what is my error ?
3. Which is the quickest way to generate the 128x100 preview pictures of the prefabs ?
Thanks for your patience with me !