Single-engine add-on
Maybe I can help. 14 CFR 61.129(a) sets forth the eligibility requirements for a Commercial pilot certificate with a single-engine rating (I assume you want your Commercial).
Sec. 61.129 - Aeronautical experience.
(a) For an airplane single-engine rating. Except as provided in paragraph (i) of this section, a person who applies for a commercial pilot certificate with an airplane category and single-engine class rating must log at least 250 hours of flight time as a pilot that consists of at least:
(1) 100 hours in powered aircraft, of which 50 hours must be in airplanes.
(2) 100 hours of pilot-in-command flight time, which includes at least --
(i) 50 hours in airplanes; and
(ii) 50 hours in cross-country flight of which at least 10 hours must be in airplanes.
(3) 20 hours of training on the areas of operation listed in §61.127(b)(1) of this part that includes at least --
(i) 10 hours of instrument training of which
at least 5 hours must be in a single-engine airplane;
(ii) 10 hours of training in an airplane that has a retractable landing gear, flaps, and a controllable pitch propeller, or is turbine-powered, or for an applicant seeking a single-engine seaplane rating, 10 hours of training in a seaplane that has flaps and a controllable pitch propeller;
(iii) One cross-country flight of at least 2 hours in a single-engine airplane in day VFR conditions, consisting of a total straight-line distance of more than 100 nautical miles from the original point of departure;
(iv) One cross-country flight of at least 2 hours in a single-engine airplane in night VFR conditions, consisting of a total straight-line distance of more than 100 nautical miles from the original point of departure; and
(v) 3 hours in a single-engine airplane in preparation for the practical test within the 60-day period preceding the date of the test.
(4) 10 hours of solo flight in a single-engine airplane on the areas of operation listed in §61.127(b)(1) of this part, which includes at least --
(i) One cross-country flight of not less than 300 nautical miles total distance, with landings at a minimum of three points, one of which is a straight-line distance of at least 250 nautical miles from the original departure point. However, if this requirement is being met in Hawaii, the longest segment need only have a straight-line distance of at least 150 nautical miles; and
(ii) 5 hours in night VFR conditions with 10 takeoffs and 10 landings (with each landing involving a flight in the traffic pattern) at an airport with an operating control tower.
14 CFR 61.65(d) sets forth the aeronautical experience requirements for an instrument rating:
(d) Aeronautical experience. A person who applies for an instrument rating must have logged the following:
(1) At least 50 hours of cross-country flight time as pilot in command, of which at least 10 hours must be in airplanes for an instrument -- airplane rating; and
(2) A total of 40 hours of actual or simulated instrument time on the areas of operation of this section, to include --
(i) At least 15 hours of instrument flight training from an authorized instructor in the aircraft category for which the instrument rating is sought;
(ii) At least 3 hours of instrument training that is appropriate to the instrument rating sought from an authorized instructor in preparation for the practical test within the 60 days preceding the date of the test;
Discussion. Most of these requirements you've fulfilled already during your military flying and via your ATP. You have to have a Private certificate or higher to apply for a Commercial Certficate. 14 CFR 61.123(h). You certainly meet that requirement with your ATP.
It appears you have to meet the ten hours of solo in a single-engine airplane requirement, the three hours of prep requirement, and the five hours of instrument training requirement. Of course, some of the dual items can be doubled up in flights, e.g., you can go out with your instructor to do commercial maneuvers and return to the field under the hood to hold, shoot approaches, etc. I don't think you have to take the Commercial written because of your ATP.
You can knock out the three hours of the 14 CFR 61.65(D)(2)(ii) instrument training as part of the five hours you need under 14 CFR 61.129(c)(i). You would have to take instruction in and demonstrate all the 14 CFR 61.127 items, which would include the maximum performance maneuvers, flight at critically slow airspeeds, maximum performance and soft-field takeoffs and landings, flight by reference to ground objects, etc. You would not have to take the ten hours of solo and practice in the "complex" airplane because you've demonstrated that already as an ATP. I added the cross-country requirements, but I really don't think you have to meet those for the same reason.
Conclusion. The idea is to cover only the operational items needed for a single-engine airplane and not to repeat items you have already from other flying, e.g. cross country. You would need to go FSDO and obtain a student pilot certificate, and have it and your logbook signed off by your instructor for the solo privileges you need until you are rated. You might be able to find an examiner who will give you both practicals in one oral and one flight. Hopefully he/she won't charge twice.
Hope this helped a little and wasn't too lengthy or confusing. Good luck with adding your ratings.
