And the problem is you have to replace an existing car to get a new car. Means you are gonna loose one of your car from your garage.
But I found a way to fix this. First add the car whatever you want ti MyCars garage. It's free of charge. After that open your save file using the above save editor. Now you can see under carrier all your cars including cars you added in MyCars.
Now select the car which you want to add to the carrier and click update. Now save & close.
Now you can see that that car will be in your garage. If you want you can add the same car again to the MyCars free of charge.