Was told that if the car doesn't already have the lights installed, one can not install them later....is this true?
I guess it depends on your state's laws,..but why they would ban properly aligned (not pointing into the eyes of an oncoming driver) fog-lamps, or mounted on a roof-rack :-) I dont know.
There's no technical or electrical reason, a long as they are fuse-protected. Some state's require some additional lights to be switched thru the vehicle's high-beam circuitry (on or off)
Personally, I haven't experienced any additional help from fog-lamps. Maybe, other cars may see you better in fog tho.
I'm sure it's not LITERALLY true; I can't see why you couldn't bolt fog lights onto anything, one way or the other.
However, depending on who was telling you (dealer?) it may not be esthetically or financially practical. The XLE comes with factory fog lights; if the situation is the same as with my '97, duplicating that setup on another variant would require replacing the entire front fascia, plus buying the lights, brackets, switch, etc. as service parts - an expensive proposition.
If NOT duplicating the factory setup, the front design doesn't really allow a lot of good looking or practical locations.