Actually, it's easy to add more vehicles.
1.) Locate files
The files needed to be modified are inside the GTA IV directory, under the common and pc folder.
2.) Edit vehicles.ide
Copy the line of the vehicle closest to the one you are adding. For instance, if you are adding a second police car, copy the entire line beginning with "police,". Or, if you are adding a van, copy the line beginning with "burrito,". Then change the first two lowercase names with the names of your model (.wtf) and texture (.wtd) files. To change the handling of your car to an already existing car, change the first vehicle name in capitals to the desired car handling id. However, if you want to add your own handling, change the name to something different. It does not matter what you change it to, but it is recommended for the name to be the same as the model and texture files for the sake of organization.
3.) Handling
Skip this step if you have chosen an already existing vehicle's handling.
It can be difficult to create your own line from scratch, so it's best to find one online. Some vehicle models come with their own handling lines. To input your new handling line, begin the line with the same name as the name specified in vehicles.ide inside handling.dat
4.) Car Colors
Inside carcols.dat, add your vehicle model name using the index at the top of the document to determine the desired color.
5.) Car Group
Place your vehicle model name in the desired group, to stop it from spawing in the incorrect location.
6.) Car Model
Use a .ide editor like SparkIV or OpenIV to add the model and texture files to vehicles.img.