Hi Ayush!

Yes, it is possible to write the NIOS after passing 12th board exams.

NIOS allows you to apply again in a subject, which you have already passed, for improvement of performance but only once. Passing 12th NIOs is equivalent to pass the 12th class form any other board (National/State/Private). You can visit the link given below to know more about the NIOS and to fill the form to register yourself for exam:

https://nios.ac.in/student-information-section/evaluation

Thankyou!