An updated version of Zanoza Modeler 2 (ZMod) has been released allowing for the import and export of DFF model files in San Andreas. ZMod has consistently been one of the more popular tools used for modeling cars and buildings in all of the games in the GTA3 series but the newer version does have some limitations. Player models cannot be exported nor imported using ZMod, and light node support is not yet implemented. Support for these things should be added in the near future. One of the more limiting features however is that the tool will only import models if the software is registered.
In order to extract or import DFF files you will need an IMG file editor. The vehicle textures can also be edited using additional software such as TXD Workshop.
Discussion on car modding can be found on the boards.