It is very easy to have more than one pilot model in the game, and it is the exact same procedure as the rest of the 3rdW series flight sims. First of all, you need to extract the pilot lod file from the ObjectData.cat file. When this is done you need to rename it and the new bitmap you have painted to a filename of your choice. Now, you need to get yourself a hex editor (this is a tool that can edit the lod files, a simple google will get you one.) With the hex editor you need to open your newly named lod file and do a text string search to find where the bitmap paint file is listed. When you have done this you need to type in the name of the new bitmap over the existing one. When you have done this, save it and place both the new lod and bitmap in the aircraft folder.
The last stage is to edit the data.ini of the aircraft you want your new german pilot in. Wrench's site has a good tutorial on how to go about doing this....
http://wrench1smog.com/wrench14.html
Hope this helps you.
As for the gas shell mod, I wouldn't know.