Joseba,
What licence are you looking to convert?
If it is just PPL, than you need to apply with FAA HQ in Oklahoma city for conversion (find out more at
www.faa.gov). You will also be require to choose a FSDO (flight safety district office). The FAA will only convert the licence once they have authenticated with the authority that issued your JAA PPL. For authentication purposes, you will be required to apply with the licensing authority that issued your JAA licence so that they can send an official letter to FAA. (Currently UK CAA charges GBP 10). All this takes about 7-10 working days. Once approved you will be required to go to FSDO for conversion.
You cannot add any new ratings to this converted licence. If you like to get commercial licence, you will be required to start from PPL written exam, flight test and move your way up.
I hope this helps.
As for job in the USA, if you are not a green card holder/american citizen you can forget about the jobs here (sorry didnt mean to sound this harsh). Presently you cannot do any new type rating (>12,500 lbs) in the USA if you are not an American citizen. For non American citizen the process is very lengthy and time consuming. The dept of Justice was supposed to come out with some new regulations for the same on the 17th March.
Good luck with your flight training.
Jatin