I don't know how to extract them, but you need to extract them in a format that meshing programs can handle. The most used and most program-friendly (but not neccessarily the easiest to work with) is OBJ, but some programs can also handle 3ds and various other formats.
I don't think there are any tutorials that show how to do this in the TS2 community, but if you can find out how to extract the meshes (Youtube or Google may be helpful) it's possible someone can help you from there.
I think a couple sites may have models from various games (possibly also GTA), for instance Vovilla.
Converting cars isn't the easiest task because it involves bone assignments and quite possibly some tinkering with textures and such.