I am assuming you are referring to the idea that IF you are rejected from a school you want to go to, then IF you receive the NSF GRFP award the university will then convert you from rejected to admitted, IF they have room.
Seems pretty late in the admission season but I would suggest that if you are rejected AND you receive the award that you reach out to those universities and inquire as to if any vacancies have opened up while somehow weaving into the email/conversation that you just found out you received the award. It couldn’t hurt to try... worst that can happen is you stay in rejected status.. you really would have nothing to lose.
Luckily the award will follow you wherever you go so as long as you get in somewhere you’ll have it to use... I haven’t read about receiving the award but being rejected from all grad programs.... that would be counterintuitive since one would suspect that if your profile is quality enough to receive an NSF fellowship then you should also be good enough for a grad school... unless you just had unreasonable expectations about where you applied and didnt apply to any safety schools.
[edit] just saw you are accepted into some universities but I’m guessing you have your eye on Harvard and MIT???
You may want to look at if they will accept an NSF award as an enhancement to any financial package you have already received or if it will be in place of... and get it in writing/email so they cant go back on it if its an enhancement situation