simran, there is no way to take NCLEX exam without getting approval from the State Board of Nursing in the U.S. It is a U.S. nurse licensure exam, so you have to do the credential evaluation before you can even appear on an exam. But if your nursing college in Nepal is kind enough to give some more additional hours than you might not have to go to school in the U.S. I have seen people going to school because they need psychiatric hours and I have seen people who didn't needed any additional hours. So it all depends.
As a first step, you have to do a credential evaluation from CGFNS (It's a globally recognized organization). Go to www.cgfns.org/ for more information. Open an account, choose the state from which you want to take an exam, pay the charge, typically it will take at least 4 to 6 months to be complete but sometimes it might take long. In the mean time when you do all this processing, work part time and read for NCLEX exam. It's pretty difficult. Good luck.