Forget about the FAA Commercial, it is quicker, easier and cheaper to go straight for the ATP.
14 CFR 61.153(d)(3) allows you to take the FAA ATP checkride while holding an ICAO CPL or ATP.
[(3) Holds either a foreign airline transport pilot license with instrument privileges, or a foreign commercial pilot license with an instrument rating, that--
(i) Was issued by a contracting State to the Convention on International Civil Aviation; and
(ii) Contains no geographical limitations.(e) Meet the aeronautical experience requirements of this subpart that apply to the aircraft category and class rating sought before applying for the practical test;]
The entire regulation is here:
FAR Part 61 Sec. 61.153 effective as of 10/20/2009
Make sure you meet the requirements, take the single written test and go for it.