Originally Posted by tallpilot

The ACS added several topics to the initial multi-engine ATP oral exam. You have already completed that. The only questions for a single add on will be about the systems on the airplane used for the check ride and performance calculations. It is a much easier ride than commercial since there are no chandelles or lazy eights but you will have to do a power off 180. Not all DPEs are authorized to give ATP ratings but most who are will have single engine authority. No written test is required. No instructor endorsement is required but of course a few practice sessions would be advisable. Any single engine CFI is authorized to give the training but I would find one with ATP experience.
Will it help you get a job? Who knows. I suspect the caravan and Pilatus operators are more concerned with total time than the ATP since it is easily added during the initial 135.297. On the off chance you get a pink slip trying to get the rating on your own that will probably damage your career prospects more than not having it. Consider the risk/reward.