Does anyone know where I can find the reg regarding adding a ASEL commercial rating to someones certificate that already has a commercial cert for a helicopter?? I've looked up and down in the regs and cannot find it!

Thanks!

That's a different category of aircraft. That's why you're having trouble.

61.129

(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.

Although it is an add-on...I'm almost positive these still have to be met since category is different.

As someone who went through this, I can tell you that as long as the person has 100TT in powered aircraft, you can definitely do everything in the 50 hours required to be in airplanes. In fact, the individual will probably end up having to fly some extra flights to get up to 50 hours (they'll be ready for the checkride sooner).

Check out your Commercial PTS. I believe that will help as well.

Yes, there's a table near the front the lists the tasks you need to perform based on your existing rating...look for the "RH" column. That's rotary-wing/helicopter.

You are adding an additional category rating (defined in part 1.1) to a commercial certificate. The reg trail starts with "61.63(b) Additional aircraft ratings (other than on an airline transport pilot certificate); Additional category rating. An applicant who holds a pilot certificate and applies to add a category rating to that pilot certificate: (1) Must have received the required training and possess the aeronautical experience prescribed by this part (in the case 61.129) to the pilot certificate for the aircraft category (in this case 'airplane') and..."

This leads to 61.129 which spells out the flight training and experience requirements that must be in airplanes - 61.129(a) - as stated above. As Rick said the actual tasks to be accomplished on the checkride are spelled out on page 1-v of the Commercial Pilot PTS under the column heading of "RH".