The easiest way is just to copy the .tweak from a similar vehicle, changing names where appropriate. (open it with notepad.) Then tweak it manually from there. It takes longer - because with the editor you can test changes as you go without having to reload bf2 each time. But if you know what you're doing it's no harder than using the editor.
I think I also need to edit the con- and ai-files(for SP), right?
I see a big challenge in getting the correct Wheel-, Turret- and Weapon-positions. Any Ideas?
That's all done in max. (you do need the editor if your vehicle is in maya format). Just hit export, it'll create the meshes and the con files, with everything in the correct positions.
It's the things you add later like cameras and weapons that are the problem.