You can add the mod to an existing save if you want. You just won't get the Mechanic Trader POI. You can either gen a new world with the mod already installed, or go into the world editor and place the POI in an existing world if you want. But the POI isn't required, just a fun addition.
And just a note, my vehicle mod does not add anything to the world other than the POI. Several people seem to confuse it with Vehicle Madness and its wrecks and repairables that it adds to the world. My vehicles are vanilla in how they are obtained.