This is more of a theory. In a save editor you can customize cop cars. But not in the actual game. So maybe the feature to customize certain vehicles was there before?
It was there before. And Rockstar left it in the game's code. Hense why a save editor exists and me, and others are aloud to do such actions.
You can also store the Stinger GT and Annihilator helicopter in Single Player, too. Whereas normally, it's just for Online.
Not only can you mod police vehicles, you can also mod boats, planes, helicopters, and the blimp. You can also store ANYTHING in a hanger or on a helipad. Like a car which goes in a garage fits right inside a hanger or on a helipad. The large Titan from the military base can also be put onto Trevor's helipad and easily can take off at his airfield and be landed. All the items you put onto a helipad or in a hanger will also respawn WITH the mods on it at all times. You'll never lose what you store there, essentially, unlike how sometimes (not for me, though) you can lose vehicles inside a safehouse or 4 car garage due to Rockstar's coding and bugs/glitches. But like I said, I don't lose items ever, not sure why people still have the issue of disappearing vehicles...