The validation or 'piggyback license' or 61.75 (all the same) is for recreational purposes only.
You get a PPL with an IR if you have passed the 'Foreign Pilot Instrument Written Test'. Any other ratings you have can be added also.
If you want a stand alone US certificate, and this is what it sounds like, you need to take a checkride.
What you should have read before you started is the following:
14 CFR 61.153(d)(3)
CFR Title 14 Chapter I Subchapter D Part 61 Subpart G Section 61.153prev | next..14 CFR 61.153 - Eligibility requirements: General.
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§ 61.153
Eligibility requirements: General.
To be eligible for an airline transport pilot certificate, a person must:
(a) Be at least 23 years of age;
(b) Be able to read, speak, write, and understand the English language. If the applicant is unable to meet one of these requirements due to medical reasons, then the Administrator may place such operating limitations on that applicant's pilot certificate as are necessary for the safe operation of the aircraft;
(c) Be of good moral character;
(d) Meet at least one of the following requirements:
(1) Holds a commercial pilot certificate with an instrument rating issued under this part;
(2) Meet the military experience requirements under § 61.73 of this part to qualify for a commercial pilot certificate, and an instrument rating if the person is a rated military pilot or former rated military pilot of an Armed Force of the United States; or
(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;
(f) Pass a knowledge test on the aeronautical knowledge areas of § 61.155(c) of this part that apply to the aircraft category and class rating sought;
(g) Pass the practical test on the areas of operation listed in § 61.157(e) of this part that apply to the aircraft category and class rating sought; and
(h) Comply with the sections of this subpart that apply to the aircraft category and class rating sought.
The ATP practical test standards are here:
http://www.faa.gov/training_testing/...-S-8081-5F.pdf
The last pages have a matrix about what you can and cannot do in which type of simulator.
Alternatively you could fly a light twin in the USA.
Good luck and let us know if you need anything else.