Enroll in a course of training for the full FAA PPL certificate.
Meet all the pre solo requirements of 61.87(d), complete the night training requirements and be allowed to fly solo at night under a solo night endorsement.
Big difference here that you are now flying on a FAA Student Pilot Certificate (& medical) and with a solo endorsement.
You are NOT flying on the priviliges of your piggy back 61.75 certificate.
This would only allow you to fly solo ie no passengers.
Rather than do the piggy back think its a good idea to get the standalone FAA licence. This does require a flight test and the US exams, but this is not much more than the checkride that any school will put you through before they will rent you an aircraft. If you have the required number of night hours in your logbook then your US licence will be valid after dark. These night hours can be done in the UK or in the US.
You get the temporary licence straight away after a successful test!!