Go through the requirements for an add on PPL, comm ASEL, and ATP ASEL in the applicable PTSs and part 61. Check 61.165.e for ATP add on info. Your checkride requirements for the ATP ASEL are easier than the PPL/COMM checkride imo since there aren't any real maneuvers, minus having to do the instrument approaches, but those are easy. It'll be no different than your ATP AMEL ride but with no single engine stuff (well I guess the whole ride is single engine, but you know what I mean) and it'd be in a less complex plane.

Don't assume that a lower rating/privileges means an easier or less complex checkride or preparation process. In this case, the opposite is true. You'd never have to do private or commercial single maneuvers. An army helo only IP/SP buddy of mine busted his ASEL private add on because his short field landing was out of PTS standards. How would you like to explain busting a PPL add on ride after being a commercially rated pilot and military instructor/examiner.

The private ride is a basic airmanship ride, the commercial ride is an energy management stick and rudder maneuvers ride, and the ATP ride is mostly another instrument ride. It's assumed at that point you've done all the other stuff. If you're already an ATP rated airplane pilot for two-motored planes, why cut yourself short on fewer privileges with more basic flying tasks to spend time/money on (and get evaluated on and possibly bust)? There's an argument to be made that any ATP ASEL pilot should be able to perform short field landings, chandelles, lazy 8s, etc., but if you never have to be evaluated on them in a jeopardy event because you hold a higher rating already, who cares?

https://www.pilotsofamerica.com/comm...l.88061/page-2

This thread is about the same issue you have and what the guy did about it (ATP ASEL add on ended up being the easiest route for him).