Sorry to have to correct you Mike, but you most definitely can add an IR to a 61.75 certificate. Indeed I have such a certificate with Instrument and Single Engine Sea privileges added with 'US test passed'.
It requires going through the TSA approval process, taking the exam, getting the required training and experience and taking the check-ride.
One good feature of the US system is that you can practice 'under the hood' without an instructor, just a safety pilot, and you can both log the time. And all your practice approaches are paid for by the US taxpayers.