If you are very much interested in studying biology you can apply it in NIOS & study from there on the other hand continue with your computer science.

Hello there. Hope you are doing good. I would like to tell you that it is upto school's decision whether to allow a student to give a particular subject as an additional subject. You can contact the school faculties or concerned person who can help you in this. You can opt for Biology as an additional subject if you want to study both Computer Science and Biology.

I hope this answer helped you. Good luck for your future.