Yes, it is possible. But it isn't easy.
First of all you need a 3d model and skins, then import these into S3D, edit the .sim, .zon, .val, .dsd and .cam files and finally edit the upc file.
With having a good 3d model and skins it would still take me something like 6-10 hours to finish it. And I've done things like this a dozen of times so I know how to do it and know what tiny mistakes will cause a CTD.
I don't want to discourage you if you want to make one, but it would definitely be smart to ask some advice. If you do you should be able to make one great x-craft
