I don't know all of your details, but if things are the way you describe them..
THEY ARE WRONG.
You can add an Instrument rating to a US restricted based on a foreign license.
Once you have the IR you can use the restricted (based on foreign license) to apply for your FAA CPL.
The only restriction to your "restricted" license is that it is based on a foreign license and all restrictions of the foreign license apply.
It holds no restrictions as to adding ratings.
14CFR 61.75 in the FAR-AIM.
Show them the regulation.
Call the nearest FAA office, they are in Tampa, Orlando and Miami.
The phone numbers are available on the FAA website at
www.faa.gov.
If they are still not convinced change schools.
*** I deal with conversions from foreign licenses on a daily basis, and the amount of sheer rubbish that people are being told is amazing ***