You might struggle with Helicopter Adventures/Bristow Academy. There is too great a demand from their own students to allow their aircraft to disappear for hour building purposes. (And they are mostly S300s anyway).
Emanuele,
I suggest that you first decide which part of the USA you want to fly in then start googling for R22 flight schools. There are lots and lots and lots. Then it will be a matter of phoning and emailing to establish who will offer you the best 'block booking' rate.
Get your JAA PPL converted to an FAA PPL (or just sit the FAA PPL flight test when you get there), get the required visa and away you go.
Of course, this matter is probably already covered in the Training thread so I suggest you have a look there first. I think that there is also a sticky in the Private Flying forum outlining the various visa and licence requirements.
Best of luck.