Looking for DPE advice. Have ATP with multiple types but no single engine privileges. Want to add ASEL (FAA)

Hello everyone. I have a question for those who may have a better understanding of regulations and certification standards.

Originally I obtained all my pilot licenses in the UK on what was then JAR, then it changed to EASA.

After many years of airline and G/A flying. I had the opportunity to come to the USA and fly in the airlines here.

When I converted my UK ATPL into my FAA ATP certificate. I did the checkride on an A320 simulator. The result was a FAA ATP with AMEL privileges. I have flown 3 different types for 3 different airlines here so my ATP now has AMEL, CL-65, BD500 and B737 type ratings on it.

I would like to be able to rent and fly single engine airplanes with my wife and kids. Essentially adding ASEL to my ATP certificate.

Whenever I go to a flight school, the CFI’s
there or the personnel seem very confused about how I have an ATP but no ASEL. They don’t really know the process to add the rating.

I have been told to contact the FSDO. Whenever I call on the phone I always get a voicemail. I have written 2 emails to the FSDO which have gone unanswered.

Can someone help?

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