Yes you can write the NCLEX exam in Saipan.
But you must first apply to California Board of Nursing after
you have received your final transcripts. Be aware that it is taking California about 6 months to send you the ATT letter (authority to test).
Also be aware that California office for processing of petitions for green cards is running about 13 months behind, and it keeps getting longer.
You cannot apply for Visa Screen either until you have completed your schooling and actually have your transcripts in hand.
While you are waiting, I suggest that you complete the set of English exams, either TWE, TSE,and TOEFL, or the IELTS series in the academic branch with the speaking section. And it would probably be to your benefit to write the boards in your country, Visa Screen actually prefers it and it will save you time. And this way you can get experience while you are waiting for your green card.
Hope that this helps....................
and remember, do not register with Pearson-Vue for the exam until you
have received your ATT letter.