can a 1968 skylark add air cond if it came off the line without it?

It depends on how much trouble you want to go to. If you don't have a console I think you can easily put an underdash unit in that car. Take a look here

for some ideas of what can go under the dash. As far as the compressor and stuff, the best way would be to go to a pick a part junk yard and look for a car with factory air and get all the pulley's, compressor, brackets, condenser, and possibly radiator, shroud, and fan. Theoretically you could pull all the factor air stuff for the dash, the evap, etc but I think you'll find it would be quite a project, you'd need a good bit of the inside parts too.

It would be easier, and possibly no more expensive, to just sell one of your existing cars and buy something with factory AC but I'm guessing you don't want to do that.

I installed factory A/C on a 95 Kia. It already had the air vents in the dash - same as a car with A/C. So just pulled all the other parts needed from the wrecking yard. That included the heating/vent/A/C control panel.

Also very important to have electrical diagrams for the car. I don't know about non-fuel injection cars, but with all fuel injected vehicles, there is wiring connected which electrically increases the engine idle speed when the A/C is on. Other switches will shut off the A/C if the high side pressure gets to be too high or if it is too low (refrigerant leaked out).