At Riva's forum, not the website, there are some games converting tutorials like the GTA IV one indeed.
Here it is, it's required the registering to read it.
Before start, it's needed a registered version of ZModeler 2.2.1 version or higher; it must be registered to enable the importing feature, required to import the GTA IV car model file. Then, you need to get the .wft file of the car model from the game files, and if you don't know how do it search about it; obviously you have to look for its textures too.
Since it's a car of the game, maybe it's possible find it converted for GTA San Andreas, and if so, the converting process gets easy.