Dupliate and simply change the name at top to any of your liking. For instance, "altaparking" and then add in vector3 coordinate of where the garage pop up will show up and you can add multiple spawn points as well.
How to change impound location?
You need to go to st-garages/shared/config.lua
Simply change and put your coordinates and it'll automatically change. If you want more impound lots, just duplicate and change the coordinates.