If you want to fly It IFR then you will need the FAA instrument add-on.
that being said it’s just as easy to take the FAA ATP written and do an ATP flight test assuming you meet the ATP flight time minimums in the FAR’s part 61.
when I moved over to the USA from Australia the most efficient and non-rip off school to foreigners was Cloud 9 helicopters in West Palm Beach Florida. The owner is a Brit expat and looks after guys switching stuff over.
if you just want to fly VFR on your FAA license then as pointed out above just renew your FAA medical and have flight review done with an FAA instructor. Good to go.