Go to your main GTAV folder then go to the lspdfr folder>data folder and select duty_selection. I edit it in notepad. In between the vehicle selections tags you can add the new lines for your add ons from the top. I think the lines from top to bottom is how it will appear when you go in the garage (first line will be the first selection in the garage). I didn't do that though, I might go back in and adjust it but I'm fine for now. You might find the agency you want and then copy that line and paste it as a new line and just change the Description Name and the Full Name to however you want to describe the vehicle and that will show in the garage. Don't change the agency ref, just put the right vehicle for that agency.