That will never work.
You can change the icon and the name by yourself and that's the only way how to get custom vehicles or those which do not exist. Either the vehicle is for everyone or no one.
The amount of people adding random cars would blow up the databases for no reason. There is more behind it then just adding a new vehicle to the game. All dependencies have to be tested, those vehicles have to be added to the missions, they need a price and their 'rescue value' to solve said missions. And that's not even everything.
You'd open a method to cheat, since you could do what ever you want. If everyone could decide what they want, the game would loose it's purpose. You cannot give the responsibility to the user, that's not how it works.